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WHEN the end came it was quick-and very final. The ground beneath the sacred city of Herakleion wobbled then seemed to melt away. The city that had been the gatekeeper on Egypt鈥檚 Mediterranean coast sank into the mud and vanished beneath the waves. A few centuries later, it was the turn of Canopus, just along the coast. The land beneath the city鈥檚 eastern suburb appeared to dissolve, and then houses, temples and roads slid into the morass.

Founded in the waning days of the Pharaohs around 2500 years ago, Herakleion and Canopus were flourishing commercial centres during centuries of Greek rule. Strategically placed at the mouth of the Nile, the two cities controlled traffic upriver and grew rich on tolls from merchant ships. Now, Herakleion lay at the bottom of the sea and Canopus was split in two-one half still standing on a rocky promontory, the other underwater.

No one left an eyewitness account of the events that destroyed these two bustling cities. Once every last stone had slipped from view, people began to forget them. Eventually no one could even be sure where they had stood. Then, last summer, underwater archaeologist Franck Goddio put the lost cities back on the map. Now he鈥檚 trying to solve one of the great mysteries of Mediterranean history: what terrible disaster wiped them from the face of the Earth?

Goddio, Director of the European Institute for Underwater Archaeology in Paris, already had a string of successes under his belt. His high-tech explorations had uncovered the submerged royal quarters in Alexandria harbour, and Napoleon鈥檚 lost fleet, sunk by Nelson during the Battle of the Nile in 1798. In 1996, he began to search for the sunken cities. Following up the few clues left in ancient texts and occasional finds from the past century, Goddio criss-crossed 150 square kilometres of Abu Qir Bay in a catamaran bristling with electronic detectors. Sonars mapped the seabed looking for any strange lumps and bumps that might be bits of ruined buildings. A seismic profiler created images of what lay under the mud. And nuclear resonance magnetometers picked out magnetic hotspots-often a sign of buried stonework. At the most likely spots, divers took a closer look.

The detective work paid off. In 1999, Goddio found the eastern suburb of Canopus 1.6 kilometres offshore. It lay under 6 to 8 metres of water and was partially buried in mud. The following year, to general amazement, he discovered the ruins of Herakleion, 6.5 kilometres from land and covering an area 1000 metres long and 800 metres wide. 鈥淵ou could hardly see the coast,鈥� says Goddio. 鈥淏ut there I was sitting in a boat above a city with everything in its original place.鈥�

As Goddio鈥檚 diving team began to excavate the ruins, incredible things emerged from the silty sea floor. At Eastern Canopus there were parts of a temple and the remains of a building 60 metres long with walls 2 metres thick. There were gold coins and jewellery, and fantastic statues including the basalt head of a pharaoh and the headless body of the goddess Isis. The objects dated from the last days of the Pharaohs to the early Islamic period in the seventh century AD.

This spring, the team began to excavate Herakleion. Out of the mud came a sphinx鈥檚 head, three colossal pink granite statues-a Pharaoh, a Queen and H芒pi, the Nile god of flooding-and remains of the Greek temple of Herakles that gave the city its name. And leaving no doubt that this was Herakleion, divers hauled up an immense granite slab bearing the city鈥檚 name. For Goddio the biggest thrill was discovering 10 wrecked ships in what was clearly the city鈥檚 harbour. 鈥淲e were astonished. They were all in a row, each still with its two anchors either side,鈥� he says.

The archaeological discoveries were spectacular, but the great puzzle remained: what cataclysm had claimed the cities? There has been no shortage of suggestions, from earthquakes and tsunamis to rising sea levels. But whatever the truth, it lay hidden among the sunken ruins. Goddio decided to call in two geological detectives to help in the hunt for clues.

Amos Nur, a geophysicist at Stanford University in California, is an expert on ancient earthquakes. He analysed the divers鈥� maps of each fallen stone, statue and column, looking for patterns that might indicate quake damage. Geoarchaeologist Jean-Daniel Stanley of the Smithsonian Institution in Washington DC is more of a mud-and-sand man, a world expert on the shifting landscapes of river deltas. His task was to probe the sediments of Abu Qir Bay looking for an explanation for the sudden demise of the cities.

Nur and Stanley agree on the basic facts: the buildings collapsed suddenly and the ground dropped by up to 8 metres. But what they don鈥檛 agree on is the cause. Nur thinks earthquakes were responsible. Stanley blames floods.

The cities vanished at different times. Herakleion was almost certainly gone by the beginning of the first century AD. The most recent objects Goddio and his team found date from the first century BC. Eastern Canopus lasted longer. The youngest objects found at the site are two gold coins, one dating from 731 AD and the other minted between 724 and 743 AD, suggesting the city survived into the Islamic period.

Signs of sudden collapse are obvious at both places. 鈥淔rom what we鈥檝e been finding at Herakleion, it looks like the city was abandoned very fast,鈥� says Goddio. 鈥淓verywhere you find gold jewellery and bronze vessels-things that are so valuable you hardly ever find them except in tombs. If there was an orderly evacuation, people would have taken these things with them.鈥� Statues, columns and walls all toppled on the sea floor and at least some of the townspeople were killed by falling masonry. At Eastern Canopus too, columns, walls and parts of temples lie tumbled on the sea floor. And with so many gold objects left behind people must have left in hurry-or were killed before they could flee.

Eastern Canopus was clearly hit by more than one natural disaster before it disappeared. Seismic profiles show several fissures beneath the ruins, one of them several hundred metres long and two metres deep. When Goddio鈥檚 team began to excavate here, they found that the huge crack had been repaired then built on. 鈥淭he city was destroyed and built again and then it disappeared into the sea,鈥� says Goddio.

Nur is convinced that earthquakes were responsible for toppling the towns and pitching them into the sea. Nearby Alexandria has a well-documented history of earthquake and tsunami damage. There are written accounts of eight quakes between 320 to 1301 AD, and there are signs of damage in the remains of the ancient city. Nur says the quakes that hit Alexandria must have damaged Herakleion and Canopus too. Unfortunately the dates don鈥檛 tally with the disappearance of the two cities.

鈥淭he physical destruction of the buildings and monuments looks like quake destruction,鈥� says Nur. 鈥淭he puzzle is the enormous subsidence.鈥� But such unstable marshy land is highly vulnerable to slumping. A massive quake could send slivers of land sliding slowly and almost horizontally into the sea. 鈥淭he ground is so soft, it would almost flow,鈥� says Nur.

But to cause such devastating subsidence the quake would have had to have been big-at least magnitude 8.5-says Nur. It鈥檚 hard to see where that would come from. The nearest seismic hot spot is several hundred kilometres away, just south of Crete where the African plate plunges beneath the European plate. This couldn鈥檛 deliver a big enough quake to the Egyptian coast, says Nur. 鈥淚t would have to be a fault nearby.鈥�

Nur suspects one of two minor fault systems might have done for Eastern Canopus and Herakleion. One runs northwest from the Red Sea through the Gulf of Suez towards Crete. The second runs from Malta, along the Libyan coast towards Cairo, passing close to Alexandria. As far as anyone knows neither fault has produced a massive quake, but as Nur points out, 鈥渕inor鈥� faults can generate huge quakes. In 1811 and 1812 the biggest quakes ever recorded in the US struck along the New Madrid fault in Missouri. No one would ever have suspected the fault could unleash such destruction because it was so far from a plate boundary.

Great leveller

Stanley suspects something altogether different. To him, all the evidence points to the Nile as the great destroyer. When Canopus and Herakleion were built, the geography of the Nile delta was very different from today. The river鈥檚 main western branch flowed into the Mediterranean at Abu Qir. This branch snaked about, shifting from east to west and back again before eventually silting up and disappearing from the landscape. Today, the Nile鈥檚 westernmost branch is 26 kilometres to the east at Rosetta.

During 15 years of research in the Nile Delta, Stanley has traced the shifting course of the Nile as far as the coast. Goddio鈥檚 invitation to join last year鈥檚 expedition meant he could follow the old river channels even further, out into the bay. 鈥淚n some places you can still see the channel as a depression in the sea floor,鈥� says Stanley. Where sediment has filled in the channel, seismic profiles picked up its course.

The ruins of Herakleion and Canopus are clearly clustered along the margins of the ancient river channels. They had stood on marshy promontories perhaps no more than a metre above sea level. The shifting river would frequently flood over its banks, inundating wide areas close to the cities. And as the floodwaters washed over the soggy marshland they would pick up a heavy load of mud and silt. Stanley thinks this could have triggered catastrophe.

The sediments beneath the cities are soft and as much as 20 metres deep. They would have been permanently waterlogged-perhaps half their volume was water. Usually, this water is safely trapped in the pores of the sediment. But the extra weight of silt-laden floodwaters can trigger liquefaction-a process in which the layers of soil are so compressed that the pore structure breaks down, forcing the water upwards in a rush. As water erupts, the ground heaves and bulges, sinking in places and forming domes of mud in others. 鈥淪ome buildings would sink down and others would be pushed up,鈥� says Stanley. Once the upward rush of water is over, the sediment can be reduced to half its original volume. 鈥淔looding can cause failure of the sediment at the river mouth. There鈥檚 plenty of evidence of this from other deltas around the world.鈥�

But did it happen here? At Eastern Canopus, there is good evidence that floods were responsible, says Stanley. Goddio鈥檚 seismic profiles provide a three-dimensional image through the mud. Normally the sediment would lie in neat horizontal layers, with the youngest at the surface. The profiles show that the layers have been badly disrupted. In places the sediment had been pushed up into domes several metres high. 鈥淎ny wall or structure there would be toppled,鈥� says Stanley. Tellingly, the profiles showed significant disruption only along the edges of the river channels. 鈥淚f a quake had hit the region it wouldn鈥檛 just select out the area along the river but would disrupt the whole bay,鈥� says Stanley.

To find out more, Stanley took cores at the spots where the sediment was most disturbed. Long sections that should have had distinct layers were totally mixed up, a sign of liquefaction. Radiocarbon dating of organic material trapped in the mud just beneath the ruins provided more evidence of catastrophic upheaval: some of the material was more than 5000 years old.

鈥淚 have a strong feeling about this one,鈥� says Stanley. 鈥淭hat failure was caused by flooding. This type of failure of the sediment happens commonly at big deltas. It鈥檚 not unique to the Nile.鈥� He has found identical features at the mouth of the Mississippi. Along the seaward edge of the delta, the weight of sediment causes faulting similar to the long cracks beneath Eastern Canopus. 鈥淎nd this is not a seismically active area. It鈥檚 not caused by quakes.鈥�

Stanley has even identified the flood he thinks was responsible for finishing off Eastern Canopus. Armed with a date-the gold coins from the eighth century are a good guide-Stanley checked out Egypt鈥檚 flood records. From the time of the Pharaohs, the Egyptians recorded the annual flooding of the Nile. The highest point of the flood was marked on 鈥淣ilometers鈥� at Aswan and Cairo. After the Arab conquest in the seventh century, the records were written down. The records show that there was a big flood in 741 or 742 AD, when the river rose more than a metre above the normal high water mark. The previous flood of this size was in 719 AD-much too early. The next big flood wasn鈥檛 for another 63 years.

As yet, it鈥檚 impossible to say much about what happened at Herakleion. Carbon dating of samples from the boats and organic material in cores should narrow down the date of the disaster. Results are expected in the next few weeks. But the precise nature of the catastrophe might take a little longer to identify. Stanley has just begun to analyse cores taken around Herakleion this spring. 鈥淛ust because Eastern Canopus was damaged by floods doesn鈥檛 mean the same thing happened at Herakleion-but flooding would be a strong contender,鈥� he says.

What surprises Stanley isn鈥檛 that the ground collapsed, but that anyone would ever have built a city in such a dangerous place. 鈥淚f you saw these cores, you wouldn鈥檛 want to build your house there.鈥� It鈥檚 not unusual for the sediment at the mouth of a river to collapse during a powerful flood. 鈥淭he ancients would have known that, so you would think they鈥檇 have thought twice about it. I guess it must have been a very profitable place to be.鈥�