On Egypt's northern coast, where the Nile delta meets the sea, there once stood two cities of such wealth and grandeur that they were famous throughout the ancient world. Today, their remains lie buried beneath a shallow bay.

Around 500 BC, the ports of Herakleion and Eastern Canopus were thriving trade centres, the gateways into Egypt for Greek ships passing down the Nile. These cities were also important religious centres, their temples attracting thousands of pilgrims each year. Yet until recently, almost all that was known about them came from ancient texts. The Greek historian Herodotus wrote in the 5th century BC that they looked like the islands of the Aegean, but set amid a marsh.

Inspired by such accounts, French businessman and archaeologist Franck Goddio began surveying an area a few kilometres west of the Nile delta called Abu Qir Bay. Goddio has been running underwater excavations at nearby Alexandria since the early 1990s. But Abu Qir may yield even more exciting results.

In 2000, Goddio revealed the discovery of two sets of ruins, including walls, temple remains, columns and statues, buried in sand under 7 metres of murky water. The first set of ruins lies 1.6 kilometres from today's coastline. In annual excavations there since 2000, Goddio's team has retrieved coins, amulets and jewellery from as far back as the 6th century BC. A slab of black granite (pictured above), inscribed with a tax edict, names the city as Heracleion and it is signed by Pharaoh Nectanebo I, who ruled from 380 to 362 BC.

The team has also identified two temples, dedicated to the Greek hero Heracles and the god Amon (the Greek version of the Egyptian deity Amun). Just to the north of the Heracles temple, the divers discovered many bronze objects, which were probably dropped into an ancient waterway as offerings. "We have found ritual deposits made by the priests in quite precise places that were known only to them," says Goddio. "We feel as if we are penetrating to the heart of the ancient liturgy."

A few kilometres away, the second set of ruins has been identified as Eastern Canopus. There, divers have discovered ceramics, parts of temple doors, coins with the profile of Cleopatra, who ruled Egypt in the 1st century BC, and a black granite shrine from the 4th century BC, dedicated by Nectanebo I, which boasts a series of astronomical and calendrical inscriptions.