Statues recovered in Aboukir Bay on Egypt's northern coast.ALADIN ABDEL NABY / REUTERS

An Ancient City’s Demise Hints at a Hidden Risk of Sea-Level Rise

Two millennia ago, an earthquake liquefied the ground beneath the Egyptian port of Thonis-Heracleion.

Sometime in the third century b.c., an earthquake struck the eastern Mediterranean. In Thonis-Heracleion, past its peak but still one of Egypt’s greatest ports, the ground began to shake, and the soil gave way. The city had been built upon low-lying islets, bits of silt and clay left behind from the Nile’s summer floods. Temples would have towered over the city, where each year, priests would form the earthly body of Osiris—the god of the afterlife and rebirth—from gold, barley grain, and river water.

In the smallest part of a second, the mud on which the city stood would have turned to liquid. The great temple to the supreme god Amun-Gereb fell into the sea.

The city did not vanish entirely that day, but without the temples, it lost its raison d’être. By the eighth century a.d., the last of its mud islets had slipped beneath the waves as the river shifted and the sea level rose. The city passed into the realm of rumor and myth. Herodotus, the Greek historian, wrote that Helen and Paris had visited before sailing to Troy. Stelae half-buried up the Nile mentioned it. A scroll found far to the south preserved a hint of its tax records. It was an antediluvian world barely more solid in history than Atlantis. No one knew where it was.

Then, in 1933, a Royal Air Force plane rattled over a bay where the Nile meets the Mediterranean. Glancing out of the cockpit, the pilot saw shapes where there should not have been shapes—the murky contours of huge stones and toppled statues, stranded four miles from the shore. It would take another 66 years for the city to be found.

Thonis-Heracleion was a victim of soil liquefaction, which is more or less what it sounds like. What seems to be solid ground in an instant melts into a roiling sea of dirt, which behaves almost as if it were water. The city carried every major risk factor for this type of disaster: It was built on loosely-packed soil which was heavily saturated—the by-product of both a high water table and recent flooding. On top of that soil, incredibly heavy structures had been built. And it was in a seismically active area, abutting the long Hellenic arc, a subduction zone where the Mediterranean joins the Aegean. Today, few places are as ripe for catastrophe—but many carry risk factors for disaster.

Soil liquefaction is a horrifying phenomenon. Videos of its occurrence look like found-footage documentaries of the Second Coming: Buildings seem to simply slip away, the earth gives out and the once-steady structures slide into the morass. But the science behind this phenomenon is straightforward. It tends to occur in loosely-packed soils—silty areas near rivers, infilled harbors and reclaimed land, marshy regions—that are highly saturated, often due to poor drainage conditions. Then add weight: buildings, roads, anything large and heavy that an engineer (or anyone else, for that matter) would not want to move suddenly.

Most of the time, nothing will happen. The saturated land will bear the weight, the soil will hold steady, and life will go on. But when pressure is exerted abruptly on the soil, combined with the weight from heavy structures atop it, the ground will stop behaving like a solid material and begin behaving like a liquid. The most common instigator for that sort of pressure is a seismic event—an earthquake.

Read: When an earthquake hits next door

Quakes cause the exact sort of back-and-forth motion required to raise the water pressure within the ground, which causes the weight of the buildings and roads above to be borne instead by the water—and water cannot support large buildings. That causes collapse, essentially instantaneously.

“People aren’t prepared like they should be,” says Dan Ander, the vice president of Washington Survey and Ratings Bureau, an insurance ratings agency. “I don’t know if people understand liquefaction. They understand earthquake damage—I don’t know if they understand how that damage happens.”

Read the full Atlantic article . . .