Here is an article called Vast underground ocean discovered on Jupiter’s largest moon. Somehow they can tell by the way the planet bulges out that there is water underneath the surface.

NASA’s Hubble Space Telescope has the best evidence yet for an underground saltwater ocean on Ganymede, Jupiter’s largest moon. The subterranean ocean is thought to have more water than all the water on Earth’s surface.

This reminded me of Stanislaw Lem’s novel Solaris. About half that book is verbal description of an ocean on an alien planet. It doesn’t sound exciting, but I found it fascinating. This is a case where a picture is not worth a thousand words, because he describes these fantastical geometric shapes that you can picture in your mind’s eye, and yet they could never actually be drawn. It’s even more amazing that the book was written in Polish, and the version available in English is supposedly an English translation of a French translation of the original. Or maybe that has something to do with why the language is so fascinating. I wish I had a copy to pull out a good quote but I don’t have one at the moment.