It would seem that Thonis-Heracleion used to be a very important port to the Ancient Egyptians, dating back 2700 years. Like Atlantis, it was doomed by flooding, and then took centuries to find again.
The mystery of Thonis-Heracleion was eventually uncovered by divers in the 21st century. At first, they thought they'd just found some submerged statues, but further dives located bridges, temples, and other buildings. What's unclear is how it came to sink in the first place. Perhaps a gigantic tsunami?