> Why would a civilization even be terrestrial, wouldn't an ocean living civilization be even more likely?

There's a book about this, can't remember the name right now. Basically it looks at very ancient maps of the Antartica that have details that we only figured out in the 20th century with equipement allowing us to see through the ice layers.

The book posits that there was a rather advanced civilization living before the last glaciation.

It also posits that all the massive flooding mythology found in most human religions is actually stories transmitted by survivors of the previous era.