Canopus and Thonis-Heracleion are interesting to historians due to their location during a time of cultural cross-pollination. Key trading ports, they would receive boats from Greece and gradually developed an immigrant community, who built their own temples and worshiped their own gods.

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By Thomas Page, for CNN

Updated 0914 GMT (1714 HKT) May 5, 2016

(CNN)Until 2001, two of Egypt's greatest cities were missing. Then along came French scuba diver Franck Goddio, who made an extraordinary discovery underwater.

For 1,000 years, Thonis-Heracleion was completely submerged. Fish made their homes among the rubble of mighty temples; hieroglyphs gathered algae. Gods and kings sat in stasis, powerless, their statues slowly withdrawing from the world, one inch of sand at a time. Goddio spent years surveying this find, as well as neighboring Canopus, which was rediscovered by a British RAF pilot in 1933 who noticed ruins leading into the waters.

Thanks to a new exhibition at the British Museum, Goddio's incredible finds will soon be open to the public.

Sunken Cities: Egypt's Lost Worlds opens May 19, and according to museum curator, Aurelia Masson-Berghoff, the exhibition pulls back the curtain on what was once one of archeology's greatest mysteries.

"(Thonis-Heracleion and Canopus) were known from Greek mythology, Greek historians and Egyptian decrees, and now we know where they were."

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Stele of Thonis-Heracleion, inscribed with the decree of Sais and commissioned by Nectanebos I (378-362 BC).

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Why did they sink?

Likely founded around 700BC, Thonis-Heracleion and Canopus acted as major trade hubs between ancient Egypt, Greece and the wider Mediterranean, located as they were at a handy intersection. But circumstances ultimately conspired against them, explains Masson-Berghoff.

"Several natural phenomenon caused these cities to sink by a maximum of (32 feet) below the sea," she says, noting that a naturally rising sea level, subsidence and earthquakes (which ultimately triggered tidal waves) all played a hand.

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Masson-Berghoff explains they also learned a lot from the form taken by the religious idols dug up from their watery grave. The statues were mainly of Ptolemaic gods with human features that represented the same qualities Egyptians prescribed to animals

"The Greeks were not exactly into animal-shaped gods nor into animal worship," she explains. "The Ptolemies, the Greco-Macedonian rulers of Egypt after Alexander the Great, created a human-shaped version of a very old Egyptian god, the sacred bull Osiris-Apis. In its 'Greek' form, he became Serapis, combining the aspects and functions of major Greek gods."

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The colossal head of a statue of Serapis will be on display alongside other large objects that blur the lines between Greek and Egyptian civilizations.

"We will show in 'Sunken Cities' a variety of sculptures depicting these Greco-Macedonian rulers as Egyptian Pharaohs, wearing Egyptian crowns and acting as if they were Egyptian Pharaohs," the curator says.

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It was not vanity that prompted their change in style, but shrewd political maneuvering. "The Ptolemies really understood that they needed the support of the local priesthood and population, to legitimize their rule," Masson-Berghoff argues. "To achieve this, they adopted Egyptian beliefs, rituals and iconography."

 

Colossal statue of Hapy, made from pink granite and over five meters high.

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The largest item on display is a statue of Hapy, ironically the god of flooding. Over 16-feet tall and weighing 12,000 pounds, the pink granite sculpture dates from the sixth century BC, long before Thonis-Heracleion disappeared into the sea.

Also worth noting is what Goddio's team left on the seabed. The archeologist discovered 69 ships: "the largest assemblage of boats ever discovered," Masson-Berghoff claims -- likely used on a Grand Canal which ran between Canopus and Thonis-Heracleion, upon which a sacred barge made of sycamore would travel during the Mysteries of Osiris, a celebration of the god of the underworld.

All of this, however, is just a drop in the bucket.

"What you need to know is that Franck excavated less than 5% of this site," the curator stresses. "They left a lot of material on the seabed." read more ...cnn