The City of Atlantis, if it exists.
There is a great deal of circumstantial evidence to suggest that Atlantis was a real place. In fact, Plato's account, which tells us that the legendary island and its capital city, were destroyed 9600 BC, has matching information from science. Three pieces of data, each from a different discipline tell us that a world-changing event occurred 9620 BC. One of those items involves sea levels, which had been rising constantly for thousands of years because of Ice Age melt, suddenly dropped by 2 meters worldwide. This could only happen naturally by the massive subsidence of a body of land somewhere in the oceans of the world. The coinciding dates make Atlantis the likeliest suspect.
I think it would prove fascinating, with the right underwater equipment, to investigate this civilization which died 6000 years before our own civilization began.