 At that time, most historians believed that Thonis and Heracleion were two separate cities, both located on what is now the Egyptian mainland.The ruins submerged in the sea were located and explored by the French underwater archaeologist Franck Goddio in 1999, after a five-year search.Numerous finds from the site have indicated that the city's period of major activity ran from the 6th to the 4th century BC, with finds of pottery and coins appearing to stop at the end of the 2nd century BC.