Heracleion , also known as Thonis , was an ancient Egyptian city near Alexandria whose ruins are located in Abu Qir Bay, currently 2.5 km off the coast. Its legendary beginnings go back to as early as the 12th century BC, and it is mentioned by ancient Greek historians. Its importance grew especially during the waning days of the Pharaohs—the late period, when it was Egypt's main port for international trade and collection of taxes.
Heracleion was originally built on some adjoining islands in the Nile Delta, and was intersected by canals. It possessed a number of harbours and anchorages, was the sister city of Naucratis, and was superseded by Alexandria.The city had a large temple of Khonsou, son of Amun, who was known to the Greeks as Herakles.Later, the worship of Amun became more prominent.
Heracleion flourished especially from the 6th to the 4th century BC, as revealed by numerous archaeological finds. Pharaoh Nectanebo I made many additions to the temple in the 4th century B.C. The city sank in the 3rd or 2nd century BC, probably due to liquefaction of the silts on which it was built following earth tremors. The ruins submerged in the sea were located by the French underwater archaeologist Franck Goddio in 2000. Until then, the scholars were not sure if Heracleion and Thonis were in fact one and the same city.