Alexander the Great’s founding of his namesake city, Alexandria, on the sandy Egyptian coast in 332 B.C., is the stuff of legend. Among the stories that circulated for centuries after his death was that he was so thorough about checking to make certain it was a good location, he had himself lowered into the ocean in a glass box to look for sea monsters that might threaten his city.

Sea monsters turned out not to be a problem. Some two millennia later, however, researchers have returned to the underwater realm to discover that Alexander was not the first to build a city where Alexandria now stands. From the chips of pottery and other clues recovered in sediment cores deep below Alexandria’s harbor, it appears Alexander the Great was beaten to the punch by at least 700 years.

“Alexander the Great didn’t just come through, spread his arms and say, ‘We’ll build here,’” observes Jean-Daniel Stanley, a geoarchaeologist from the Smithsonian’s National Museum of Natural History who led the study. “He actually was expanding a previous settlement.”

Stanley didn’t originally go to Egypt to search for lost cities. For more than 20 years, he has studied sedimentation in the Nile Delta and has visited Alexandria numerous times to investigate two very real problems facing many coastal cities—subsidence and sea level rise. A large part of Greek and Roman Alexandria is now under water. Stanley was trying to learn how it happened.

“Did it occur slowly over time by natural processes as the land sank and the sea level rose?” Stanley asks. “Or did the influences of brief catastrophic events, such as tsunami or earthquakes, play a part? Did humans play a role by overloading water-saturated sediments with buildings to the point of failure?” Answering these questions may have implications for modern-day New Orleans and Venice, two cities that are similarly situated below sea level.

In 2001, Stanley and his team sank seven hollow metal tubes, just under 4 inches in diameter, as deep as 18 feet into the sediments of Alexandria’s harbor. Radiocarbon dating revealed that the cores penetrated deposits laid down more than 7,000 years ago—millennia before Alexandria was built.

Examining the cores of material pulled up inside the tubes, Stanley observed that the old deposits contained some surprising and unexpected features. Pebbles of igneous rock that could only have come from distant quarries in Middle and Upper Egypt were among the sediments.

“They weren’t carried there by the Nile,” Stanley says. “There’s nothing else of their size in the channel. It’s all fine sand.”

Some layers were rich in organic material. But that didn’t fit either. Organic-rich sediments of this type typically form in brackish marshes. “But there were no marshes along Alexandria’s open marine bay,” Stanley points out. “It’s been marine from the word go.”

Most striking, though, were the pottery fragments that Stanley found at levels in core samples dating as far back as 940 B.C. Was it possible that chips of pottery from later times, after the founding of Alexander the Great’s city, had somehow become mixed into the older layers by movement of the sediment or by the operation of the coring drill?

Stanley consulted Gus Van Beek, a specialist on Greek pottery at the National Museum of Natural History, about the apparent anomaly. Van Beek confirmed that the pottery was locally fired, not imported, and typical of the region during the pre-Alexander era. So the fragment did appear at the correct level in the cores.

“The eureka point for this study came when I realized: I’ve got the radiocarbon dates, and there’s stuff below the date of Alexander,” Stanley says. “Doesn’t that mean there was something there before he founded the city?”

There were the stones, probably imported for building material. The high organic content probably came from waste water. And of course, there was the pottery. When a colleague suggested that he analyze the sediment for its lead content, Stanley saw an opportunity to add another piece to the puzzle. “The Greeks used a lot of lead,” he says.

With the help of geochemist Richard Carlson at the Carnegie Institution in Washington, D.C., Stanley determined that, sure enough, lead concentrations in the sediments increased long before Alexandria was founded.

Historians have generally believed that there was some kind of early settlement in the area before Alexandria. In The Odyssey, Homer praises the quality of the harbor’s anchorage.

“When you look at the map of Egypt’s Mediterranean coast, you can see that the most logical place to put a harbor is right there at Alexandria,” Stanley says. “So it is not surprising that the place would have been known for a long time.”

But until Stanley’s discovery, there was no hard evidence that this early settlement, traditionally called Rhakotis, was anything more than a fishing village or that it had persisted for so many centuries.

Only so much can be learned from sediment cores, however. Stanley hopes that his discovery will be followed up by archaeologists working on dry land to locate stronger evidence of the old city of Rhakotis. “This would be a major logistical task as Alexandria today is a densely packed city of 4 million people,” Stanley observes. “Everything is built up, and there aren’t a lot of open areas to excavate.”

Still, one area of the modern city of Alexandria bears the ancient name Rhakotis. Is it the original site of the pre-Alexandrian settlement? No one knows for certain, but Stanley believes it’s the logical place for archaeologists to begin looking.

“I don’t think they would be disappointed,” he says. “I think they would find Rhakotis.”