Pre flood civilizations before 20,000 years ago were possibly near ancient coastlines and now underwater.
Anomolies exist like Malta, the Bimini road and the ancient causeway off the west India coast. After the flood many projects sought to build refuge from rising water in mounds, pyramids, and mega structures. If the advanced cultures far ahead of hunter-gatherers were few it would not be surprising that we have not found them underwater. A handful of civic centers are few compared to billions of fossils.

SEEKING THE GOLDEN SPIKE
Even a small lake in Canada holds secrets.