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If an island in a sub-tropical region were to be unreachable because of natural phenomena, what could those phenomena be?

The people trying to reach said island have 17th century seafaring technologies.

Alex Hintermann
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    Unreachable for what reason? There is no sub tropical place on Earth humans hadn't reached in the 17th century. So why can't it be reachable? Or must it merely be undiscovered? – Mormacil Jun 11 '17 at 10:22
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    I suggest checking out the novel Fragment. Hender's Island is a great example of an isolated and unreachable ecosystem (that the book proves should definitely, DEFINITELY stay that way xD). – Z.Schroeder Jun 11 '17 at 13:33
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    You should further elaborate o why you want it unreachable. For example, an island that is submerged for most of the time is reachable, but none goes there. A good place to hide things. – Ludi Jun 11 '17 at 14:51
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    Supernovas. A nearby supernova could kill off all life on the planet, which would make the island unreachable to people with 17th century technology. – HopelessN00b Jun 11 '17 at 22:53
  • does unreachable means no written record of it being available? There could be island where people already went there, but just didn't live to tell others. – Vylix Jun 12 '17 at 02:52
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    Piranha ring. Extreme radioactivity. Supervolcano. Extreme evaporation of all the oceans on the planet. A less-than-nice sea "dinosaur". Uninhabited planet (can't be reached if there's no one to reach it). Huge meteor impacting the island and destroying it. Huge meteor impacting the planet and destroying it. Options galore! – xDaizu Jun 12 '17 at 07:06
  • I added more details above, as to why the island is unreachable. – Alex Hintermann Jun 12 '17 at 07:39
  • Do the inhabitants of this world know the island exists? Like there are legends about it but no one can get there? – Wes Sayeed Jun 12 '17 at 08:11
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    Don’t edit the question in a way that invalidates existing answers! – JDługosz Jun 12 '17 at 10:16
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    Indeed, I do not want to invalidate the other answers, as many of them are awesome. Where can I put a refinement of the question then? In this comments strand? (Noob. Sorry.) – Alex Hintermann Jun 12 '17 at 10:27
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    ADDITIONAL INFO: For my purposes, the island should be unreachable for a meteorological reason. Something to keep its ecosystem isolated and a sea-faring empire out. The island itself is quite large and habitable, with jungle, volcanic mountains, and desert. – Alex Hintermann Jun 12 '17 at 10:27
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    @WesSayeed The residents of the empire have descended from the people on the island, but too many generations have passed for them to remember. – Alex Hintermann Jun 12 '17 at 10:34
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    How big an island, some of small high cliffed ones are unreachable by anything but helicopter. – John Jun 13 '17 at 04:44
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    @John The island is large, about 50 square km. – Alex Hintermann Jun 13 '17 at 06:52
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    @AlexHintermann, feel free to add new information to your question just mark it as such. Just start the new information with a bolded "edit" title, to keep once valid answers from getting downvoted. Helping you to refine the question is part of why comments exist. – John Jun 13 '17 at 14:28
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    Lousy cellphone reception. – Hot Licks Jun 14 '17 at 01:43
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    Put it in the center of a permanent hexagonal storm, like the one in saturn – The One Jun 14 '17 at 15:40
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    Unreachable is a question of degree. Just how motivated and well-financed are the people trying to reach this island? Do they have just one ship with ordinary equipment, or is this a major expedition sent by a world power to take the island at all cost? – David42 Jun 14 '17 at 19:28
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    When the Mongols under Kublai Kahn tried to invade Japan by sea, they were defeated by a variety of factors, including typhoons. The storms became known as the Kamikaze (神風) or Divine Wind. Strictly speaking the storms alone didn't prevent the Mongols landing, but they are an interesting historical real world example of a relevant meteorological event. – kojiro Jun 15 '17 at 11:08
  • @AlexHintermann You say 17th century seafaring technology is available. How are the other tech levels? – Weckar E. Jun 15 '17 at 13:09
  • @kojiro I would say the most significant of the factors you mentioned was not the kamikaze, but rather the Chinese boat builders who, having great knowledge of the weather patterns in that area, made ships that would be capsized by waves offshore. – chiggsy Oct 05 '18 at 21:15

21 Answers21

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Reefs.

enter image description here Public Domain, USGov-NOAA, 2005-07-25

They are and were a very dangerous obstruction for ships if they only leave very shallow water for passage. Because the ground climbs very steep, it causes massive breakers, destroying any ship stranded on the reef and making a passage per boat very dangerous.

ADDITION:
In fact, there is a reason why most of the exploring of uncharted lands occurs during the 18th century. Only precise navigation allow the creation of maps which give location of reefs for further exploring and that was not possible until the longitude problem was solved.. James Cook had access to the newly available lunar distance method and later copies of John Harrisons H4 chronometer.

MAKING IT MORE DANGEROUS AND IMPENETRABLE:

Put the island in the subtropic southern pacific. There both trade winds and the South Equatorial current are running in the same direction. Fully-rigged sailships like in the 17th century can run close-hauled (zig-zagging in direction of the wind), but not very good. So if the island is a far away from ports in the west (logistics), it is nearly impossible to reach from the western side. On the eastern side build reefs/shallows which are formed like a elongated horseshoe. Large reefs like that reveal themselves through the continous braking of waves so the sailors will be alarmed in time, but the ship is trapped in lee shore position: Both wind and current are moving it against the reef which is a death trap. Only agile ships like schooners will be able to escape the trap.

Thorsten S.
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    It would have to be hidden reefs that sink ships, or miles and miles of them that stop you getting out your big boat into a rowing boat to get ashore. – MrLore Jun 11 '17 at 15:59
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    Any barrier reef has a deep-water connection to the ocean -- tidal flows will prevent an unbroken ring from forming, or will quickly erode it open if it does form. Any captain worth his salt, upon approaching such a reef, would have someone up in the mast looking for the passage and someone with a lead-line in the bow taking soundings for it. A reef is tricky but not impassible to 17th century ships. – Mark Jun 12 '17 at 02:01
  • @Mark Couldn't a natural breakwater allow a large reef to survive? For example if the unreachable island was in the middle of an archipelago that protects it from large surface waves that hit the islands further out? – MrLore Jun 13 '17 at 07:25
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    @MrLore Then you have a place to anker your big ship and reach the island with shallow boats. – Angelo Fuchs Jun 13 '17 at 17:15
  • @AngeloFuchs Hence why I mentioned miles of unbroken reefs which you couldn't pass with rowing boats. And if surface waves are reduced, they can grow to a shallower depth without being eroded which may prevent any form of water craft passing it. – MrLore Jun 13 '17 at 22:16
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    @MrLore Catamarans existed as early as 1500 BC. A small, light catamaran could be used in place of a rowboat to sail over miles of reef. – Brian McCutchon Jun 15 '17 at 06:40
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Consider Rockall.

enter image description here Source: Wikipedia.

Rockall is a barren island that is approximately equidistant between Ireland, Scotland, and Iceland, temperate, but with no natural harbor or even an obvious place that one can land without ramming into a cliff face. Wikipedia, citing Fisher, James (1957). Rockall. The Country Book Club. pp. 23–35., states that the first known visit to Rockall was in 1810.

Robert Columbia
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  • It might not be hospitable but this island is certainly reachable. I'd assume the question wants an actual meterological argument instead of the issue being purely geological. – Garto Jun 11 '17 at 17:15
  • @Garto I guess if you make it enough of a cliff it is unreachable? – MCMastery Jun 11 '17 at 19:01
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    @MCMastery We had climbing gear even in medieval times (although rarer than today). – John Hamilton Jun 12 '17 at 07:05
  • @JohnHamilton Part of the problem is getting stable enough to disembark when the waves are moving the ship up/down/against the rocks without smashing your ship – fyrepenguin Jun 12 '17 at 07:27
  • @fyrepenguin people can swim though, you can just take a small boat and row close enough, jump in the sea, secure a nail or two then keep going up. (Unless it's really bad weather and the waves are huge) – John Hamilton Jun 12 '17 at 07:32
  • @JohnHamilton Yeah, but make it larger, with higher cliffs, along with some severe waves, and you'd be in business. Especially combined with any of the other answers here, like a reef or underwater rocks. Kill off adventurers before they reach the island, then prevent people from even landing in the first place, and with no place to dock a boat, seafaring empires would be very unenthused about going there. – fyrepenguin Jun 12 '17 at 07:43
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    @JohnHamilton Though I'm not going to deny the sheer tenacity and stubbornness of the human race, and people would still find a way to get there – fyrepenguin Jun 12 '17 at 07:45
  • @fyrepenguin just because it might be possible doesn't mean that it has been done. There are Tepuis that have not yet had anyone walking on them. A scaled up version of this could be no different. – Baldrickk Jun 12 '17 at 10:57
  • Agreed, @Baldrickk. I was mainly saying that this answer, possibly with some tweaking, should keep people out for a while, but eventually (possibly post-time period that the OP is interested in) a determined enough person will get there. I figure that the determination is a major factor; natural barriers will deter, but not stop enough determined people (especially with modern tech). 17th century people though, should have great difficulty though, without killing a lot of people in the process, if they succeed at all. – fyrepenguin Jun 12 '17 at 11:38
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    @Garto I understood the OP to mean practically inaccessible. Today, humans could theoretically go almost anywhere in the Solar System, except for that there is simply not enough interest and money for doing so. If there was a large golden statue on the top of Rockall for all to see, surely the 17c Royal Navy or some other deep-pocketed searing force could have thrown resources at it until a landing could be made. With an estimated cost/benefit for landing on Rockall at a net loss, rational people, including even many grimey stereoptypical 17th century pirates (arr), are going to pass it by. – Robert Columbia Jun 12 '17 at 19:26
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    Looks reachable to birds. – Colonel Panic Jun 13 '17 at 08:37
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    @Baldrickk: well, wikipedia says there are 115 Tepuis in the world. I barely can imagine that even one of them had not yet a human beeing walking on them. Even not yet as they are. – Zaibis Jun 13 '17 at 11:15
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    @Zaibis announcement of a BBC2 documentary following climbers attempting to access one that had not been visited before: http://www.bbc.co.uk/mediacentre/latestnews/2016/steve-backshall-adventures I saw the documentary when aired, they failed. They said quite clearly in the documentary that there are more that have not been climbed, and unlike Auyantepui, there is nowhere to land on these (on Auyantepui, there are flat exposed rocks where the river reaches the edge. They were shown flying there at the end of the documentary) – Baldrickk Jun 13 '17 at 12:33
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    Lol, they should have called this island Fuckall, because that's really more descriptive of what you'll get for actually succeeding in climbing it. :) – fgysin Jun 14 '17 at 08:42
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An island that is entirely surrounded by an undersea methane gas fissure that is constantly active would be inaccessible by any kind of ship even with modern seafaring technology. The only way to reach such an island would be by air (which your world doesn't have).

Methane gas has the very real-world property that it reduces the buoyancy of a water causing any boat passing over a gas fissure to capsize and sink.

Such fissures do exist naturally on the Earth. In fact, methane fissures are one of the more plausible explanations for the Bermuda Triangle (even though the Bermuda Triangle is myth; ships are no more likely to sink there than anywhere else).

Update:
If you're looking for a weather-related phenomenon, think of a permanent hurricane that encircles the island at all times. Jupiter's Red Spot is an example of a hurricane that has been spinning for eons. It's not hard to imagine something like that existing on a habitable planet.

If the island were in the eye of such a hurricane, the winds there would be calm and the environment peaceful. None of the inhabitants there (if there are any) would know of the world beyond the great storm, and no outside vessel would survive the trip through it.

Wes Sayeed
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    I'm confused how something can be the most plausible explanation for something that doesn't exist. – sphennings Jun 12 '17 at 03:48
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    @sphennings; The same way there are Bigfoot 'experts' even though no there is no evidence of such a creature existing. – Wes Sayeed Jun 12 '17 at 04:06
  • Great answer! Could you link more information about this effect? (Add a link please) – Cameron Leary Jun 12 '17 at 05:28
  • I remember they talked about this at Discovery Channel, they even prooved it with a on scale experiment – Fez Vrasta Jun 12 '17 at 07:10
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    "(even though the Bermuda Triangle is myth; ships are no more likely to sink there than anywhere else)" I doubt this. There are plenty of places where ships are way less likely to sink that in the Bermuda Triangle. For instance: atop a mountain – xDaizu Jun 12 '17 at 07:10
  • @xDaizu; Heh... It's that Noah's ark? – Wes Sayeed Jun 12 '17 at 08:01
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    @xDaizu, are you claiming that the percentage of ships that sink when they are on Mt. Everest (for example) is greater than, less than, or equal to the percentage of ships that sink when they are in the Bermuda Triangle? – Joe Jun 12 '17 at 18:25
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    @wessayeed Self proclaimed Bigfoot experts aren't a reputable scientific authority. – sphennings Jun 12 '17 at 21:14
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    @sphennings; Of course not ;-) And neither are Bermuda Triangle believers. But it doesn't stop them from coming up with good, science-based explanations for why they're so elusive. – Wes Sayeed Jun 12 '17 at 21:34
  • The good science based explanation is that they don't exist. Anything else is wrong. – sphennings Jun 12 '17 at 21:35
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    @sphennings; Bigfoot - Endangered Mystery! – Wes Sayeed Jun 12 '17 at 22:26
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    @sphennings There are good, [tag:science-based] explanations for dragons. Are you sure you're on the right site? :-) – wizzwizz4 Jun 13 '17 at 06:55
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    As to the hurricane part... generally meteorologically won't work for an Earthish planet. Jupiters hurricanes move, as do Earth's. Hurricanes can certainly slow and stall. But doing so causes upwelling and cools waters, and would eventually lead to weakening. Could we change enough planet parameters to make it work? I don't know, it'd be an interesting question to further consider. – JeopardyTempest Jun 13 '17 at 13:28
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    @Joe "are you claiming that the percentage of (...) is greater than, less than, or equal to the percentage of (...)?" Since percentages are (usually) real numbers, the answer to that question is, tautologically, yes. Given two real numbers A and B, A is either greater than, equal to or less than B. – xDaizu Jun 14 '17 at 15:46
  • @xDaizu, given the particular numbers we're talking about, that's a bold claim. – Joe Jun 14 '17 at 17:03
  • Great answer! Even airplanes, if the existed, would not be able to travel safely in the area due to the large air drafts. This is why methane gas is such a popular answer to the mysteries of the Bermuda Triangle. – Clangorous Chimera Jun 14 '17 at 19:56
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    @xDaizu, technically a ship on top of a mountain has already sunk. – crobar Jun 15 '17 at 14:06
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Rockall is a good idea but lets go even farther and add some nasty vulcanism:

You have an island that has cliff faces except in one area. You can pass through a narrow, turning area and reach a calm body of water within. Volcanic gases are bubbling up in the lake. Normally it is in a delicate balance with the CO2 saturated water staying trapped in the depths. It doesn't take much to upset the balance, though--say, a ship's anchor dropping into it.

The ship sails in, drops anchor, the CO2 rises and kills everyone with no apparent cause of death. It won't take much for the people around to leave the killer island well alone. (Without an understanding of the situation they won't realize that after it strikes the threat is gone for some time.)

Loren Pechtel
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    So it's well reachable, just not leavable. – Elise van Looij Jun 12 '17 at 14:06
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    @ElisevanLooij Unleavable implies impassable. Put it around the island, and the island itself becomes unreachable. – Brilliand Jun 12 '17 at 22:09
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    @ElisevanLooij Actually, they would never actually reach it. The ship sails in, drops anchor and lowers a small craft to actually go ashore. The CO2 boils up, they die before ever setting foot on the island. – Loren Pechtel Jun 12 '17 at 23:01
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    If your rising gasses produce a sufficiently dense bubble curtain around the island, that would even prevent the ships from floating anymore, right? That'd be a pretty effective deterrent. If the average density of the foamy water is lower than the average density of the ship, the ship will end up at the bottom. – John Walthour Jun 13 '17 at 15:41
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    @JohnWalthour That's even scarier - ships sail into the island and simply sink into the water. – IllusiveBrian Jun 13 '17 at 18:03
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    IMO this is possible even without the high cliff faces if the lagoons and bays are semi-active volcanic craters themselves - I like it. It provides a reasonable way for an island to be "unreachable" without being "undiscovered", and allows tech transfer to natives.

    If the lack of discovery is desired, placing large shallow volcanic fissures and vents further out might mean that a ship doesn't even have to anchor to have problems. Random CO2, H2S, methane, and other volcanic gasses would wreak merry havoc on shipping - either by virtue of killing everyone or sinking the ships outright.

     – Mark Roberts Jun 13 '17 at 18:07
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Doldrums, Trade Winds, Gyres, and optionally: Perpetual Overcast and Military Mandates.

Start off by having your island located in the doldrums; anywhere between 5°N and 5°S.

Next, make the topography of your island so that whenever the trade winds do kick up, all the air at the surface flows away from the island.

I'm no meteorologist but I think if you made Death Valley (which is below sea level) into an island (and perhaps surrounded it by mountains), that would condense the moisture out of the passing air and continually feed these winds. Or all of the above, vice versa.

See also, ocean gyre. For instance, the Sargasso Sea, which is the home of the Bermuda Triangle and the real reason why the area is so treacherous to sailing vessels.

enter image description here

If you really want to screw with the navigator, make your planet perpetually cloud covered. That will make using sextants difficult and navigating by the stars impossible.

Couple all of that with a mandate that under no circumstances should any naval vessel sail into these waters, and until someone eccentric enough and with deep enough pockets charters a 'scientific' expedition, your mystery island will remain completely undiscovered. Yar. There be dragons.

Mazura
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    As for perpetually overcast, the Vikings used sunstones for that with some success. – DevSolar Jun 13 '17 at 11:00
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    As a meteorologist, the idea of continually outward flowing winds surrounding the island is what came to mind first too. No idea what peculiar topography it would take to sustain it, but an intriguing idea. – JeopardyTempest Jun 13 '17 at 13:30
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The only thing that keeps people away now is radioactivity.

In the 17th century this was not something they knew about. But they would simply get sick and die terribly if they tried to stay there for any length of time, especially if they collected the strange glowing rocks.

So why would short-lived (geologically speaking) isotopes be around?

For a first idea, consider the natural fission reactor.

How about a meteorite? A chunk from a newer patch of dust, only recently enriched by a supernova, sent planitismals scattering.

Or, you might start with natural uranium or thorium ore, but provide a unique way to concentrate daughter elements to dangerous levels. People are experimenting with using microbes to mine, and metal deposits have been caused by such microbes. So maybe something evolves that uses uranium and thus concentrates it. This also causes high mutation rates, so you see other microbes and eventually complex life that makes use of all the available daugher elements. This has the further benefit of making all the life on the island “mosterous”, decended from extremophiles that live deep in the Earth and hot springs.

And of course it’s all highly toxic and radioactive: even the pollen will be inhaled and cause bleeding tumors. Any crew that comes up to the island for a few hours, even if they don't send a shore party to get fresh water and food (as is SOP) will be too infirm to work the ship within 2 months and dead in 6. Normally any ship will take on water, meat, and fruits and vegetables — this will kill them within days.

JDługosz
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    The premise was not uninhabitable, but unreachable. While radioactivity might kill people who get there, it doesn't stop people from reaching it then leaving quickly, with at least someone surviving. (Also it was 17th century in the question) – fyrepenguin Jun 12 '17 at 07:31
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enter image description here

It could be closed off on all sides such as this example from Pokemon. This is a giant volcanic crater, the only way to get in there (in this case) is by diving underwater. While diving is certainly something that most anyone could do, finding an entrance wouldn't necessarily be easy (the first people to discover it would have likely done so by luck).

JustSnilloc
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    Just made an account here just to upvote your answer. Btw may you could add that this is Sootopolis City (Xeneroville) in Hoenn, area where Ruby, Sapphire and Emerald take place. – Swizzler Jun 13 '17 at 20:31
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Voracious wood boring fauna

enter image description here

Make the shipworms very large and very hungry, and no ship will come anywhere close to the island and tell the story until a century later when they figure out copper sheathing.

Denis de Bernardy
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Perhaps a volcanic eruption or a extra terrestrial impact created a crater with several concentric ring walls. The outermost ring wall looks like a very large island with steep cliffs rising out of the sea all around.

Trying to land on the cliff side is very dangerous.

If someone manages to land and climb up to the top of the cliffs they will see the top of the ring wall is rather thin and bare and treeless. On the inner side are more cliffs falling down to into the sea, connected by underwater passages, or maybe a freshwater lake formed by rainwater.

And maybe from the top they can see all the way to the next inner ring wall, or maybe it is beyond the horizon.

So there is no gap in the outer ring wall, no way to sail a ship through it, no trees to build a boat or a raft, and it would be be very hard to haul a boat up and then lower it into the inner body of water to explore it.

Thus the outer ring wall will be mapped. But nobody knows how many inner ring walls there are and if there is a large island in the center. Does that make it isolated enough for 17th century technology?

Possibly someone made up a myth that there is a central island that is a paradise or loaded with gold or something, and thus the protagonists foolishly believe the myth and seek to reach the central island without any proof there actually is one or what it may be like.

M. A. Golding
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    Honestly, this may be the best answer, it sounds quite feasible (albeit ridiculously unlikely of course) – JeopardyTempest Jun 13 '17 at 13:33
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    JeopardyTempest - Yes ridiculously unlikely, but definitely as inaccessible with 16th century technology as could be desired. In fact the writer would probably have to put in a few gaps in the concentric rings to make it a little easier to get to the center islands. – M. A. Golding Jun 15 '17 at 22:19
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To elaborate on Z.Schroeder’s remark about Henders Island, a very complete plot outline is given on Wikipedia.

The island is populated by viscous fast monsters that have remained isolated because they cannot tolerate salt water.

The island is remote, in the South Pacific, “about 1,400 miles south-southeast of Pitcairn Island.” It’s at a latitude where ships don’t like to travel, so the shipping lanes miss it.

“The island is only about two miles wide,” Glyn said, encouraged. He read from cue cards Nell had prepared for him. “Since it is located below the fortieth parallel, a treacherous zone mariners call the ‘Roaring Forties,’ shipping lanes have bypassed it for the last two centuries. We are now headed for what could well be the most geographically remote piece of land on the Planet Earth. This empty patch of ocean is the size of the continental United States, and what we know about it is about equivalent to what can be seen of the United States from its interstate highway system. That’s how sparsely traversed this part of the world remains to this day. And the seafloor here is less mapped than the surface of Mars!” Glyn got an appreciative murmur out of the crowd and he charged on. “There are only a few reports of anyone sighting this island, and only one report of anyone actually landing on it, recorded in 1791 by Ambrose Spencer Henders, Captain of the H.M.S. Retribution.”

The 1791 voyage that did stop there found a butte sticking out of the water with no landing, as Robert Columbia notes in his answer. This includes lack of a reachable anchorage for the ship anywhere around the island.

Note that in this story, only one crewman was killed. The monsters only attacked the shore party, and the ship, several hundred yards off, was safe. The captain chose not to report the attack but indicate that it’s not worth visiting.

So, it is approachable, but nobody bothered going there. An ocean vessel could keep station without anchoring and send a launch. They could bring ropes, ladders, etc. to get up onto the land (but then be killed).

But even without monsters, his explaination as to why nobody did go there (even though they could) does seems to hold up through the age of sail.

  • location away from shipping routes
  • documented as being worthless to voyagers
  • not near anything
JDługosz
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  • "Not worth visiting" does the trick. Say the place is worthless rock. Even if someone goes there after you, if the place is inaccessible enough, nobody is going to question it. Those that do, might die trying or might agree with you after the misadventure. If 5 people over the centuries say it is worthless and there is no indication they ever came back to the island twice in their lives, then it is worthless. – jo1storm Jul 13 '17 at 08:06
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An island with no natural harbors, with steep cliffs all around (such as Rockall in a previous answer), in choppy waters and with at least seasonal bad weather, should do the trick, at least statistically. There's no place that can be one-hundred-percent-guaranteed unreachable but there are many places that most people don't consider it worthwhile enough going to great trouble trying to reach in the first place.

If your island has no safe place to land a boat, and the weather around it is usually bad, and the island is away from the main seagoing routes (so that getting there takes a lot of time), and there's nothing there of great value (that you know), then only very determined people will even thinking of trying. That in itself will also contribute to the island's legend of inaccessibility.

pablodf76
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Basically something that kills off people

or atleast deters them from going there

Here are some scenarios...

  • Strong gale winds and repeated cyclones regularly occur in a ring-like arrangement around the island causing attempts by explorers to prove fatal, discouraging others from trying.
  • Floating sea weed surrounding the island releases [enter some hormonal drug-ish something], causing sailors to be very aggressive. This leads to massacres aboard the ship, while the lone massacre-er starves to death as the weed entangles the ship's rudder.
  • Jagged rocky structures surround the island, both above and below water, severely damaging ships, while row-boats are not enough for the rest of the journey.

Your imagination limits you in these setups.

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Varad Mahashabde
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    How would such a coastline work, the jagged rocks just below the surface? Is the island volcanic? And your magic is at odds with the science-based tag. – Mormacil Jun 11 '17 at 12:25
  • Someplaces they rise above the water, blocking ships, and other places they are just below the surface, badly damaging ships. Like the ones they show in Pirates of the Caribbean 5. the magic part has been removed. – Varad Mahashabde Jun 11 '17 at 12:29
  • I get what you were going for, but I was interested in how you envisioned them geologically. To remain sharp they must be quite hard as softer rocks would be eroded round. The Science-based tag calls for science based answers. – Mormacil Jun 11 '17 at 12:31
  • Cyclones would only work in certain seasons. Most likely between July to October and even then they would not be constant. – Bellerophon Jun 11 '17 at 12:36
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Put your island near the poles. This works on two levels: one it's too cold to approach, and two it's too close to the poles for magnetic-based navigation. In fact, Captain Cook didn't cross the Antarctic circle until the 1770's, which is later than the time period you're discussing. Antarctic Timeline of Discovery

Depending on how big your island is, the harder it'll be to get there; an island only a few hundred meters across placed right at the very poles might never be discovered, especially if there's no other land masses within the Arctic/Antarctic circle for any explorers to desire to go there (from the above link, most of the early Antarctic exploration was due to hunting seals etc.) It would be much later before steamships with ice-breaking prows would be able to get close enough through the surrounding ice to deploy ground teams (dog sleds, etc.)

So:
1) Difficult to navigate to even within a few hundred kilometers of the island; compasses don't work, cloud cover for most of the year so astrogation doesn't work.
2) No need to go that far south to begin with as there's no indication there's resources there worth exploring.
3) Large/powerful ships are needed to get within range of the land.
4) The ice extends too far from the land mass to make a ground-trek to the island possible.

Nathan
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Snakes can cause problems for colonizers. As mentioned here:

A lighthouse was constructed in 1909 to steer ships away from the island, operated by a single family. The family was found dead in the 1920s, having died from attacks by golden lanceheads that had entered the residence.[5] The lighthouse is now automated.[6][7] Due to the number of snakes and toxicity of their venom, the Brazilian Navy took action and closed the island to the public.

Count Iblis
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  • I bet insects could play this role. I am thinking of the scene in African Queen where they try to tie up for the night and are nearly driven mad by the mosquitoes before they escape back to open water. – Willk Jun 13 '17 at 02:31
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There is a difference between unreachable and uninhabitable.

Johnson/Nikamuoro Island, where Amelia Earhart probably met her end, is reachable, but in a tropical climate with no fresh water, it is uninhabitable for any length of time.

Bikini Atoll is reachable, but is still too radioactive to remain for long.

Rockall Island is reachable, but there is no practical reason to want to land there, aside from the difficulty in actually getting ashore.

For an island to be unreachable, with 17th century sailing methods, and for that island to be located in the subtropics, which counts out extreme cold in the polar regions preventing it from being reached with 17th century technology, it would have to be a combination of the island itself being uncharted and thus unknown (a lot of the oceans were uncharted in the 17th century), and protected with either doldrums (no wind) or prevailing winds not being favorable for discovering the island.

Or maybe Jacob told John Locke to move the island again...

tj1000
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Just to build on some of the excellent suggestions here already (particularly the reference to the Sargasso Sea) ... I think one could really push one's creative license to describe an island that is formed from the volcanic pumice ejected from a globally-sized geo-kinetic cataclysm (see Deccan Traps).

The volcanic pumice is buoyant and naturally floats (silicon dioxide mostly)... and given a mid-ocean gyre such as the previously mentioned Sargasso Sea, one could have an island that is rotationally bound and agglomerates much in the same way that planets form from proto-planetary disks, or similarly the ocean garbage patches currently found in several oceanic locations of our own world. Sufficient geologic time and a few thousand fertilizing seabirds could instantiate a biome on this floating mass of pumice.

The ocean gyre adds climate-change possibilities on human time-scales as the surface current and trade-winds slowly but surely rotate the pumice-island from the sub-tropics to the sub-arctic and back again on regular and predictable cycles.

For a 17th century mariner, this moving target would be the equivalent of Terra Incognita, a rum-induced mirage, or similar, yet it can be habitable and perpetual… and intriguingly predictable.

apetit
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  • This opens some interesting story possiblities. Reminds me a little of the Discworld island Leshp, from Jingo. I was surprised to learn from that article that Leshp could actually have been based on the real-world Graham Island off the southwest coast of Sicily, which is a volcanic island that briefly broke the surface in 1831. Several other books by well-known authors were inspired by the event and subsequent political arguing over ownership. – brichins Jun 15 '17 at 16:47
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  1. The island is surrounded by sharp rocks, the area is shallow, and/or strong currents smash the ships into the rocks.

  2. The island is encircled by a lava, wood ships touches it, starts a fire. The burning ship has to retreat or everyone is killed by the lava or ship fire.

Even if there was a natural underground thermal vent and water temps were 140F, the crew would be in a wooden oven(ship). Leave or bake to death, even 120F might be problematic.

cybernard
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Its just really far away from anywhere worthwhile going.

The Easter Island was discovered by a European in 1722 after someone spotted it by accident in 1687. So, had it been somewhat smaller (as is your island) and also typically rainy there (lots of clouds), no one will have plausibly have found it.

Angelo Fuchs
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  • The island is surrounded by a waterfall - the sea water goes to the abyss.

  • The island is surrounded by extremely turbulent water, with whirls.

  • The island is surrounded by extremely hot, boiling water.

  • Extremely dangerous animals occupy the waters around the island, for instance those who prey on ships and their crews.

  • The island is surrounded by artificial anti-ship traps and hedgehogs installed in the ancient times.

Anixx
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  • By "iceland" do you mean "island?" Also, I'm confused, but curious, what you mean by hedgehogs. – Alex Hintermann Jun 13 '17 at 07:21
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    @Alex Hintermann https://en.wikipedia.org/wiki/Czech_hedgehog – Anixx Jun 13 '17 at 07:24
  • I was more intrigued by the idea of R.O.U.S.-style hedgehogs. :) Not sure how the other kind would keep light landing craft and personnel away; easy to bypass small anti-ship traps with a couple logs sent ahead to clear a path. – brichins Jun 15 '17 at 16:52
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Both of the following would be unreachable:

  1. An island which is the top of an erupting volcano
  2. An island which is an active geiser w/ boiling water flowing over a narrow and jagged rim

Add earthquakes, tsunamis, and lightning storms if you want (and get yourself an overkill)

jaam
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The idea isn't fully fleshed out, but... imagine an island with ring mountains with unscaleable cliffs. There is a sea current that runs around this tear drop shaped island. It has a saltwater river exiting on the leeward side of the island. This river is fed by an underwater intake that funnels water from the current, like a ramjet. Leaving the island would be easy. Fishing with nets in the flow provides for the inhabitants. The cliffs would have the added issue of extreme smoothing and polishing by sand blasting provided by the current. There would be no anchor point for a ship and smaller crafts would be pushed along by a current that got stronger as you approached the island due to a venturi effect.

Jammin4CO
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