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 have 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 Surveying and Rating 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.”
The risk factors that lead to liquefaction could be compounded further, as climate change induces sea-level rise. Because liquefaction vulnerability increases when soil becomes more saturated, rising seas will naturally cause greater vulnerability in at-risk areas and have the potential to endanger previously safe points further inland. Thonis-Heracleion experienced flooding before its liquefaction event, possibly caused in part by relatively minor changes in the Mediterranean, which has seen its sea level rise incrementally since the last Ice Age. “If you raise the water level, you raise the pressure in the water all the way down the profile,” says Peter Campbell, an underwater archaeologist. “You consequently reduce the contact stress between the particles [so] you have less margin if any excess pressure is generated by the earthquake.”