Via The Atlantic:
Helen of Troy is believed to have walked there. So is her lover, Paris. The cityknown as Heracleion to the Greeks of the time and as Thonis by the Egyptians who built it was once a central part of ancient Egypt — the port of entry to the empire for all ships coming from the Greek world. It was likely established in the eighth century BC, making it even more ancient than the so-famously-ancient city of Alexandria.
There was one problem, though. “Constructing a huge city on the bank of a river and the shore of a sea,” Nature Education puts it, “comes with a risk.” And “Thonis took that risk but ultimately paid for it with its life.”
Indeed. Sometime around the 8th century AD, Thonis sank into the sea. Most scientists believe that a combination of factors contributed to the cataclysm: a rise in sea level, coupled with a sudden collapse of the sediment-heavy earth on which the city was built.
Whatever the causes, though, the results were clear: Heracleion — Thonis — essentially collapsed into itself. The city built upon the water plunged into it.
And then it descended into myth. For hundreds of years, people assumed that, despite historical reports suggesting its existence, the city was nothing but a legend. And then, in 2000, a team of scientists rediscovered the sunken city. Led by the famous underwater archaeologist Franck Goddio, the team conducted a four-year-long geophysical survey of the area that would likely be the city’s watery resting place. Finally, they found what they were seeking. Beneath the waters of modern-day Aboukir Bay, a mere 20 miles northeast of Alexandria, the team located the remains of a once-bustling city, silenced by he waters.