Are you sure? what about fossils and other remains? So far there is zero evidence that there was an advanced or non advanced civilization. Unfortunately this nonsense is the bread and butter ot pseudo archeologists.

I mean sure, if they somehow didn't have any trade goods that went in land, didn't work with metals enough to do any appreciable mining, and didn't breed any plants or animals enough to change their genetics. Any hypothetical hidden past civilization would have to have been weird in a number of ways to not leave any evidence.

>coastal civilizations say, a million years ago, would be now under the crust.

Nah, you way off on that, coastlines have remained relatively same last million years, def not subducted into an ocean trench

Huh?

We have plenty of fossil evidence from, from your example, a million year old hominids. Far more than we would need to expect to see evidence of civilization.

Subduction makes up for a tiny tiny proportion of where coastal land "goes" in any age.

You'd need an enormous set of coincidences for a past civilization of hominids to go unnoticed.