People have been looking for ancient civilizations for a long time. Here's a clue. Look under water. Not as people are living under water, as in civilizations were slowly covered submerged by rising seas as the glaciers melted away. The oceans are about as high right now as they are going to get but 30,000 years ago the oceans were 450 lower than they are right now. Physical evidence can be found off the coast of Japan
There are probably sites like this all over the world and this my basis for that hypothesis. Something like half of the people on the earth live within 50 miles of the oceans. The oceans were much lower before the ice melted. The ancient civilizations were covered up with water. Some of the people migrated to other parts of the world and resettled bringing their advanced culture with them.