How effective would Rods from God be on an underwater target?

Question

If I take a Rod from God, drop it on to an ocean somewhere, what exactly happens?

Let's say there's a secret base about 10,000 feet or so underwater. Is it still okay?

Answer 1

Lets use the Newtonian impact approximation... penetration depth is approximately $l\frac{\rho_p}{\rho_t}$ where $l$ is the length of the penetrator, $\rho_p$ is the density of the penetrator and $\rho_t$ is the density of the target.

20ft of tungsten hitting sea water at terminal velocity might be expected to go in about 114m or 374ft. This might not be a great approximation of penetration depth, but probably isn't out by an order of magnitude.

Exactly what that would look like I'm not sure... a big splash, naturally, but most of the energy would be spent in the upper layers of the water. I'm not sure if the rod would be disrupted by the end of that 114m... probably not, so it might actually sink down far enough to go "donk" on the roof of the underwater base in an hour or so, but terminal velocity in sea water isn't quite as dangerous as in air.

The amount of energy in a RfG isn't that great... only equivalent to a few tonnes of TNT, so I wouldn't hope for the initial impact to generate a shockwave capable of rattling your underwater base either.

Using an RfG on anything on an abyssal plain is clearly a non-starter. But why use such an inappropriate weapon? Just use nuclear depth charges. Simpler, cheaper, more destructive. Don't overcomplicate things!