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Preface
With all technology discovered prior to this question, on today's planet, there are still some things we have not observed - which may be used to justify introducing new life forms, right here at home, in a story.

There are a LOT of unexplored places on Earth

To be clear

I am defining "undiscovered species" as:

  • Somewhat unique. Cannot be a subspecies or a division of a well-known species into groups that are somewhat similar to each other.
  • Unbeknownst to or denied by modern scientists. Can be seen by isolated or indigenous societies.
  • Lacking observed evidence of existence to the point that any actual evidence found is explained by some other phenomena. Like if the bloop was an animal and we brushed it off as ice moving, or like if this thing wasn't actually a known animal, or if this thing wasn't a sleeper shark.

So the question is

Given the above information, how large can each of the following organisms plausibly be on Earth in 2016 while remaining undiscovered with current technology?

  • Plant, fungus, or similar species
  • Land animal
  • Aquatic animal

*I will not accept "it will probably be..." I would like calculations or references as opposed to speculation, please.

Edit: In response to all answers - I ask "how large can they be" not "what is a size they could be". I do apprciate your answers but every one seems to say "this is a large thing we've seen so an organism could be at least this large". What no answer does say, as the question asks, is "based on the ways we observe the world, an organism could be at most this large".

Zxyrra
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    Since this is a question asking us to speculate on things that are plausible (i.e. they may or may not be true), the hard-science tag is inappropriate. You should go with science-based instead. – kingledion Nov 17 '16 at 14:26
  • @kingledion - agreed, I wanted to submit an answer, but since it's tagged 'science-based,' the only answer is "as large as the largest unexplored geography." Would be a good question, though. – Mikey Nov 17 '16 at 22:52
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    After having long been thought to be extinct, one species of Coelacanth was disovered in 1938 and another was discovered in 1999. They can get to be two meters long and 90 kg. https://en.wikipedia.org/wiki/Coelacanth Someone who looks for such species in a less than scientific way is called a cryptozoologists and some cryptids sought included at https://en.wikipedia.org/wiki/List_of_cryptids There is some evidence suggesting that may be a tribe or two of a hominin species similar to Homo Florensis on an Indonesian island. See https://en.wikipedia.org/wiki/Orang_Pendek – ohwilleke Nov 18 '16 at 00:21
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    See this. I'm amazed it wasn't discovered until a few years ago. – JDługosz Nov 22 '16 at 05:40
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    Oceans might have a ton of hidden lifeforms, but there is also the possibility that there is life under the earth as well... and we know almost nothing about sub-terrain ecosystems. In addition, there is evidence of life forms that originally are assumed to be separate but are in reality a massive root network like: http://www.amusingplanet.com/2010/11/pando-single-largest-living-organism-on.html – Phil M Nov 23 '16 at 19:11

7 Answers7

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Land animal

First, the answer depends on your definition of 'undiscovered'. If you mean 'undiscovered to Western scientific classification' then it is quite possible that a currently known sub-species will be elevated to species sometime in the future; much like the African Elephant is in the process of being split into two species (the original bush elephant Loxodonta africana and the forest elephant Loxodonta cyclotis which a recent DNA analysis indicates is more closely related to an extinct European Elephant than to the bush elephant)

If you mean 'undiscovered by Westerners in general', the winner might be the kting voar, if that is non-mythical and also non-extinct. The wikipedia page I linked is pretty skeptical, but consider that a different distinct bovid was found in the same region in 1992, the saola.

If you mean 'undiscovered by any people'; well, then it would have to be something Antarctic, or tiny, because people are everywhere. Its hard to even say what would constitute 'undiscovered' in that sense, so I'll let it pass.

Aquatic animals

The first specimen of the colossal squid was captured in 2007. Since it is only recently that the giant squid and colossal squid were distinguished from each other, it is plausible that there could be another large squid type hanging out in the deeps (Titanic squid?)

Sleeper sharks are also large and poorly known. The Southern Sleeper Shark was determined to be distinct from the Greenland and Pacific Sleeper Sharks only in 2004. The sleeper shark niche seems like a plausible place to find a new species. Another shark option are deepwater plankton feeding sharks like the megamouth shark. The megamouth has only been discovered in tropical regions; there could be a polar variant hiding in deep oceans.

Trees

Given the incredible diversity of tree species in rainforests, I feel confident that there are 30m+ undiscovered trees hiding in the darkest Amazon or Papua New Guinea.

Fungus

The giant fungus in Orgeon was hardly suspected to exist until an investigation in 2003. It seems pretty likely that such a fungus could exist elsewhere in the world without being noticed. A plausible location would be in the mountains of southern Siberia or Mongolia. They are sparsely populated and have a climate generally like that of Malheur National Forest in Oregon. Another option would be the lower Andes in Chile and Argentina.

Of course, looking for the same kind of giant fungus might blind us to the possibility of a different kind of giant fungus. Might there be huge fungal colonies collecting nutrients from rainwater washed underground from a lush rainforest? It could be in the limestone caves under jungles of Vietnam, the cenotes of the Yucatan, or even in the giant underground counter-Amazon 'river'.

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kingledion
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  • This answer is currently closest to winning the bounty as it provides much of the information I need. However it would be helpful to me if you could cite a source or calculate "at most as large as..." instead of giving examples - "at least as large as ..." Otherwise very thorough and much appreciated – Zxyrra Nov 22 '16 at 05:45
  • I'd have added a section on eusocial insects. Are vast numbers of sterile worker castes individual animals, or part of a single colony that is the mature size of one instance of its species? If you accept the latter, then the supercolonies which arise when some new ant species is introduced to a new continent, might be of note. – nigel222 Nov 25 '16 at 17:46
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    You suggested fungal colonies, but may have neglected to mention the variously known, clonal colony ( i.e. physiologically conjoined? ) specimens. https://en.wikipedia.org/wiki/Clonal_colony – Nolo Nov 26 '16 at 19:37
  • This answer is closest to winning the bounty (ending tomorrow) but it will not receive it if it fails to say "at most this" as opposed to "at least this" – Zxyrra Nov 27 '16 at 02:47
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    @Zxyrra It does say 'at most this'. – kingledion Nov 27 '16 at 12:54
  • @kingledion I see several "at least as large as" but nothing says "it couldn't be larger than" – Zxyrra Nov 27 '16 at 13:14
  • @zxyrra To ask specifically for Must be of size x or smaller for something as vague as undiscovered is unreasonable simply because of one single factor, Population. Should the animal be nearly extinct and have a very small population, it could easily be the maximum size possible and go undiscovered simply because there are so few of them thanks to random chance. So your size limit is literally the maximum, except when you consider having more than a few of them. Other criteria you do not give are critical to answering this question to the precision requested – Ryan Nov 28 '16 at 18:53
  • @Ryan Turn that into an answer in the next 49 minutes and the bounty is yours – Zxyrra Nov 29 '16 at 01:16
  • There's really no upper limit on the size of an undiscovered fungus. There are certainly bacterial colonies miles underground. There could be an enormous fungal colony in the same environment. There's no reason there couldn't be a single enormous fungus a hundred miles across, threaded through the rocks two miles underground. – Mike Scott Mar 10 '17 at 06:03
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There is an island in the Andaman Sea where the human inhabitants literally kill on sight any human being that sets foot on the island. They are living a Stone Age tribal existence, but we don't know anything about their language and culture, because anyone who tried find out got dead before learning anything.

Whatever animal species exist on that island have been "discovered" and described only by those Stone Age natives.

Just about anything might be living there. As long as it can survive alongside a tribal human society - megafauna are probably out, for example. The archaeological records show that megafauna have never survived the arrival of human beings. But anything regular-sized (up to the size of an elephant) would be completely feasible.

If you want to know the theoretical maximum size this undiscovered land mammal could be, it will depend if you want it to be a mammal or not (heat dissipation becomes an issue before biophysical weight/mass limits kick in), and whether it has live young or lays eggs. There is a summary of research into size limits on sauropods, which explains other factors like how much they chew their food and how long their necks are ...

The contraints for sea animals are admirably covered in the answers to this question.

I can't see any upper limit to the volume of a fungus, because entire aspen forests are a single individual organism. The largest on record is 106 acres, and there would be no practical obstacle to it continuing to expand. Of course, if it couldn't live in sea water, your fungus would be constrained to the size of whatever land mass it was on, and, to remain undetected, would need to be under the surface of the ground, except for its "flowers" - whatever shaped thingy it pops up to spread its spores from time to time.

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Jnani Jenny Hale
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  • How are elephants not megafauna? – kingledion Nov 17 '16 at 14:27
  • Well, @kingledon, anything over 40kg (88 pounds) could be defined as "megafauna", but that would be a pretty small elephant! The "mega" is relative to the "standard" and "miniature" versions of whatever species. I was referring to Pleistocene megafauna. The elephant version of that is the woolly mammoth. – Jnani Jenny Hale Nov 17 '16 at 16:44
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    Tracking, but you said that humans make megafauna extinct, but elephants are feasible. It seems that if humans made megafauna extinct, than maybe monkey and rodents and other not-megafauna are more feasible. – kingledion Nov 17 '16 at 17:28
  • @JnaniJennyHale saying that megafauna can't survive men while referring specifically to Pleistocene megafauna is circular logic. Obviously, no species extinct in Pleistocene is extant today - big or small. Saying that "mega" is relative to "standard" fauna is wrong too. Wolly mammoths were no bigger than modern African elephants, they are either both megafauna or none of them are. – Borsunho Nov 18 '16 at 00:05
  • The fossil records clearly show mass extinctions of megafauna in Australia, New Zealand, and the Americas, correlating with the arrival of human beings in the area. There are animals in Australia that weigh more than 40kg, but the megafauna kangaroos were much larger than the modern ones, even though some of the modern ones weigh more than 40kg. I think you are splitting hairs over terminology, because the point is clear. I you can think of a better word, feel free to edit it in. – Jnani Jenny Hale Nov 18 '16 at 05:06
  • @Jnani Jenny Hale: If the definition of 'megafauna' is only 40 kg, then in temperate North America the buffalo (300-1200 kg), moose (350-600 kg), elk (250-500 kg), and grizzly bear (130-360 kg) all easily qualify, and all exist in the wild. – jamesqf Nov 22 '16 at 05:00
  • @jamesqf, exactly my point. Modern "megafauna" like elephants would be conceivable on an island in the Andaman Sea but not the hyper-sized versions of modern-day animals that lived on continents without a human presence. – Jnani Jenny Hale Nov 23 '16 at 13:25
  • The bounty is about to expire but I do not have what I need to select any answer as of now. You may be able to win it if you address "at most this large" as opposed to "at least as large as" – Zxyrra Nov 27 '16 at 02:48
  • Does that edit do it for you, @Zxyrra? – Jnani Jenny Hale Nov 29 '16 at 04:13
  • @JnaniJennyHale Once / if it addresses plants, fungi, and sea creatures yes (see bottom of question) – Zxyrra Nov 29 '16 at 04:25
  • How about now, then? :) – Jnani Jenny Hale Nov 29 '16 at 05:10
  • You are now addressing everything but you do not explain the maximum size of undiscovered organisms in a quantative way - only the maximum size theoretically possible. – Zxyrra Nov 29 '16 at 22:23
  • I am not sure what is the difference between "the maximum size theoretically possible" and "the maximum size". In the case of plants, it is limited only by your choice of land area. In the case of ocean creatures, it is limited only by the shape you want to make the creature (it can't be too bulky - you need a lot of surface area). In the case of land animals, it depends on whether you want a mammal, whether you want it to have live young, and how you want it to process its food. Unless you supply those details, an actual quantitative answer is not possible. – Jnani Jenny Hale Nov 30 '16 at 01:49
  • @kingledion The claim was that megafauna can't survive the arrival of humans. Humans never arrived in Africa and Eurasia, they evolved in place alongside the animals, which is why the animals could slowly adapt to the slow evolution of humans and survive. Humans arrived in the rest of the world, and killled the megafauna, which had no time to adapt. Of course the Andaman Islands are in Asia, and so any megafauna there would have had time to adapt to humans. – Mike Scott Mar 10 '17 at 06:06
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I believe that the calculations you are asking for are impossible.

One reason is the difficulty of defining discovery. Take Bigfoot. Has he been discovered? Plenty of people believe in him. Or does it only count if a reputable scientist bags one and carries it off to be stuffed in a museum of natural history?

And what about species that were only recently discovered to be separate species?

Empirical evidence provides a lower bound.

Every now and then, scientists discover a new species or accept the descriptions by an indigenious people that were previously discounted.

  • A new monkey in 2010. Half a metre.
  • A new tapir in 2013. More than a metre.

From that we can state with some confidence that an undiscovered, human-sized mammal might exist.

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o.m.
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  • "Discovered" is generally not "believing in", I mean discovered as "the body is found and identified as a separate species" or "it is observed multiple times and described" which is generally the way it is done in science - not "belief"
    • – Zxyrra Nov 17 '16 at 05:22
  • If we're describing something drastically larger than what is thought to exist I doubt it will "recently be discovered to be separate (from something we know)"
    • – Zxyrra Nov 17 '16 at 05:22
  • If you accept "discovered to be separate species", then the size of a small elephant: http://www.birds.cornell.edu/brp/elephant/cyclotis/cyclotis.html It's certainly possible that such a splitting of species could happen with whales, too. – jamesqf Nov 17 '16 at 06:40
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    "Plenty of people believe in him." About as many as believe that all world leaders are lizards.

     – Feyre Nov 17 '16 at 12:43
  • Another example. Giraffe have been found to be several distinct species. We thought they were only one. – EveryBitHelps Nov 17 '16 at 14:12