Statues recovered in Aboukir Bay on Egypt's northern coastAladin Abdel Naby / Reuters

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 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.

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.”

Even the ancient Greeks and Egyptians realized they needed to change something about Thonis-Heracleion, though it’s unlikely that they understood the exact seismic or geological risks of the site. When Alexander the Great set about building a new port in 331 B.C., in the century before the earthquake and liquefaction nearby, he and his armies picked a rock outcrop, away from the Nile Delta and unlikely to move.

The team that found Thonis-Heracleion was led by Franck Goddio, a French explorer and archaeologist known for his discoveries of ancient cities and early-modern shipwrecks. He and his colleagues started searching the waters of Aboukir Bay, near Alexandria, in the early 1990s, both for Napoleon Bonaparte’s ill-fated flagship L’Orient, sunk in a 1798 battle, and for the mysterious objects the pilot had spotted six decades before.

No one knew with any certainty what it was called, merely that a pilot had seen something out there. After roughly a year of searching, the team discovered magnetic anomalies seven kilometers from the shore. The city had been completely submerged for at least 1,200 years, and the earthquake that sank the great temples had struck the better part of a thousand years before that.

Once they had located the ruins, Goddio and his team set about determining exactly what they had found. First, they saw an inscription on the collapsed wall of the city’s great temple—it was dedicated to Amun-Gereb, a localized form of the Egyptian deity Amun. The temple was known to be located in the city of Heracleion, an entrepôt that Egyptian tax records and Herodotus’s Histories attested to. Not long after, the team discovered a stele whose inscription stated that it had been erected in Thonis, another port mentioned in ancient-Egyptian records. “We understood in a fraction of a second,” Goddio told me: They had found two of the great lost cities of antiquity—and they were one and the same.

The realization that Thonis and Heracleion were one city—the crucial port on the Nile before Alexander the Great’s conquest of Egypt, where grain was shipped out and wood, gold, and silver shipped in—upended the historical consensus that two different, significant Egyptian cities were once located on the current mainland. The city’s discovery also indicated that the coastline had shifted more dramatically from ancient times to the present than had previously been understood.

Thonis-Heracleion could have been rebuilt after its religious sites were claimed by liquefaction, but evidence shows that the decline hastened after that, with people leaving the city in droves. The city was first and foremost a religious site, and though houses of worship are, often enough, built to look indestructible, even massive limestone blocks like those of the great temple at Thonis-Heracleion can prove vulnerable. “The collapse of a temple in an earthquake would have to be proof that that wasn’t the right god, or that that god was getting back at you,” Lucy Jones, a seismologist who spent three decades with the U.S. Geological Survey, told The Atlantic.

In her 2018 book The Big Ones: How Natural Disasters Have Shaped Us (and What We Can Do About Them), Jones argues that the destruction of churches and temples can be particularly devastating for societies. In 1755, she writes, one of the most destructive earthquakes in recorded history struck Lisbon and altered the psyche of the Portuguese nation, promoting a new focus on efficient state power. The city was all but leveled, first by the shaking, then the tsunami, then the liquefaction along the banks of the Tagus where the greatest buildings of the city lay. After watching his world crumble, the strongest buildings reduced to rubble and ash, King Joseph I refused to live indoors and constructed a massive tent complex to dwell in.

The cities of the 21st century are no more immovable. Most coastal communities are vulnerable to sea-level rise. Many are vulnerable to liquefaction, which in an instant can claim chunks of the most sturdily built structures.

In Christchurch, New Zealand, a 7.1-magnitude quake struck in Darfield, deep in the city’s hinterlands, in September 2010. Its aftershocks continued for more than a year, and in February 2011, a 6.3 quake hit Christchurch, almost directly in the city’s center. There were 185 deaths, and much of the city’s eastern suburbs were destroyed due to liquefaction, either in the quake itself or after, when they had been deemed unsound.

“I was in the first floor, so it was immediately obvious to me that it was extremely damaging,” said Misko Cubrinovski, an engineering professor at the University of Canterbury in New Zealand and an expert on liquefaction. He was able to begin research on areas affected by liquefaction soon after the quake, a relatively unusual opportunity. Though the Avon River flows through the city—and much of the liquefaction occurred near its banks—rockier sections of the city prevented the soil from liquefying with as much totality. Christchurch was never as vulnerable as Thonis-Heracleion. But the disaster is probably the closest thing to the calamity in Egypt that the 21st century has seen.

Christchurch has put effort into solving its problems: Stricter building codes have come into effect, and the most at-risk areas have not been redeveloped. “There are tracts of land along the Avon River that are devoid of houses,” says Ronald Andrus, a civil-engineering professor at Clemson University. “You can see the vegetation indicates old property lines, but it’s just been grassed over.” Even with the changes, Christchurch may not be doing enough. Other cities—the ones that have not learned tough lessons about liquefaction, at least in living memory—certainly aren’t. “We are still overall making decisions that are not the right ones and are creating future vulnerabilities,” Cubrinovski said.

No one knows exactly how big Thonis-Heracleion was. But the archaeological record points to a substantial city. “You have to understand that the city covered a square mile, more than a square mile. It’s not a small city,” says Damian Robinson, the director of Oxford University’s Centre for Maritime Archaeology. This spring, Robinson, Goddio, and their teams returned to Thonis-Heracleion: Less than 1 percent has been excavated, according to Robinson, and they will continue picking through silt.

One of the most intriguing objects the team has uncovered, among the roughly 75 shipwrecks and more than 700 anchors discovered so far, is a small boat—carefully scuttled and built of sycamore, a wood sacred to the god Osiris. Around it, incense burners, containers with bits of plants and bones inside, and ladles—facing eastward—were found. It may have been part of an annual ritual known as the Mysteries of Osiris, in which a boat was sent upriver carrying two forms, the god’s earthly and heavenly bodies, representing, as Bojana Mojsov, an Egyptologist and author of Osiris: Death and Afterlife of a God, says, “the link between temporality and eternity … two different concepts of time that existed simultaneously.”

Few cities today are truly like Thonis-Heracleion, but the danger that destroyed it should be more worrisome than ever. Cities on the West Coast that are vulnerable to liquefaction are relatively easy to identify—the fault lines that cause quakes are well understood, so analyzing the soil is all that’s necessary in most cases, Andrus says. On the East Coast, vulnerabilities are more subtle. While the seismic risks are lower, they are also far less understood, he told me, which leaves cities like Charleston and Savannah—both low-lying ports built on liquefaction-prone soil—in a potentially perilous position. Lagos, London, and many cities in China and Southeast Asia are all theoretically vulnerable to liquefaction under the right seismic circumstances.

And sea-level rise can increase vulnerability. Even the smallest increase in the level of a local water table can raise a city’s risk of liquefaction. If a modern-day Thonis-Heracleion were to experience floods—say, worse than usual, thanks to rising seas—and then suffer a quake, the devastation could be cataclysmic, sending dozens or even hundreds of structures crashing down within seconds. That’s the promise of sea-level rise in many liquefaction-vulnerable places. Little by little, the water table rises, and after one flood, a quake comes, and the soil turns to liquid, to a weakened goo incapable of supporting the heavy brick and steel and broad wood beams erected upon it.

And so it all collapses.

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