Homer talks of Poseidon lashing out, Plato refers to a massive marine disaster. What happened on Sardinia in the second millennium BC? What dramatic event swept away the Tyrrhenian civilisation and the “tower builders” cited by Strabo and the poet Hesiod in antiquity? Was it an earthquake or a tidal wave? A comet? Was it punishment meted out by Zeus, as Plato suggests in Critias, acting pitilessly to improve the behaviour of these people who had been spoiled by living in a land where it was always spring? Certainly they occupied a beautiful, fertile island, endowed with all sorts of metal, both hard and malleable, such as zinc, lead and silver.

Writer and journalist Sergio Frau, one of the founders of Italian daily La Repubblica, has been investigating the subject for more than 10 years, drawing on the texts of the ancients. A dozen or so Italian scientists joined him when he visited Sardinia in early June. They included historian Mario Lombardo; archaeologist Maria Teresa Giannotta; Claudio Giardino, a specialist in ancient metallurgy; cartographer Andrea Cantile; archivist Massimo Faraglia; and Stefano Tinti, a geophysicist and expert on tidal waves.

The aim was to air hypotheses just before an exhibition entitled Big Wave: The Mythical Island of Sardinia opened at the museum in Sardara. Le Monde followed them through the fragrant brush, heavy with the smell of myrtle, artemisia, rock rose and rosemary, seeking out the shade of twisted old olive trees and cork oaks, climbing to hilltop where the remains of ancient megalithic edifices found in Sardinia lie hidden.

Sardinia might be Plato’s island of Atlas, or in other words Atlantis, which the Greek philosopher placed beyond the pillars of Hercules, the strait between Sicily and Tunisia. Herodotus and Aristotle shared this view, which contradicts the idea that the term refers to the strait of Gibraltar, as was commonly supposed from the third century BC onwards. Frau, too, holds this conviction. Seen from the air the southern end of the island resembles “a marine Pompeii submerged by mud”, he says. Digging into this mud turns up ceramics, cups, pots, oil lamps, sharpening stones, metal implements, knives, chisels, needles and arrow tips, all mixed up, as if the people had been forced to drop everything and run. These remarkable archaeological finds attracted very little attention until the mid-20th century. For good reason, though. For about 3,000 years the island seemed to be under a curse, a prey to malaria until 1946-50 when the Rockefeller Foundation experimented with the use of DDT for eradicating the mosquitoes that carried the disease. We now know that thousands of nuraghi – megalithic fortresses with a central tower – are scattered all over the island. They date from the middle of the bronze age, between the 16th and 12th century BC. In Medio Campidano province, in the south, they have vanished under piles of earth covered in vegetation. Only the ones on high ground, over 500 metres, have been spared. In the past 20 years the number of registered structures has risen from 9,000 to 20,000.

There are 20 structures of this kind on the basalt plateau of Giara, the core of a volcano that now rises to about 600 metres above sea level, extending over 42 sq km. It is home to small wild horses with long manes. The towers seem to stand guard over the plain below. “In the mid-bronze age the plateau was used for winter pasture,” says Francesco Casu, a local guide. “Each tower belonged to a clan which owned the surrounding fields.” In the lowlands the nuraghi resemble pyramid-shaped hillocks. The most complex example is Su Nuraxi, at Barumini. Archaeologist Giovanni Lilliu uncovered this massive building in 1950. Some time before his death, aged 98, in 2012, he explained to Frau how he had been intrigued by a cavity everyone called the well, set on a small hump of earth and pebbles. But no one had the faintest inkling such a treasure was hidden inside.

The Barumini site, which was added to the Unesco world heritage list in 1997, is a spectacular achievement. To reach the central fortress we pass through a labyrinth of circular walls, corresponding to the houses of a later hamlet. The most striking feature is the way the huge basalt blocks forming the central tower fit together. The tower is conical, with a floor of polished pebbles, and covered by a Mycenaean-style dome. It dates from the 16th century BC – according to the fossilised olive branches found inside. Four turrets, dating from the 12th century BC, surround the main tower. They are connected by underground passages, testimony to the skill of its architects. A storage cavity keeps food at a constant temperature of 12C all year round.

Were these towers built as defence against some enemy, to house local lords, or indeed for signalling?

View from inside a nuraghe, a megalithic fortress with a central tower, at the Su Nuraxi archaeological site on Sardinia. Photograph: Valeriy Tretyakov/Alamy

Some historians suggest that messages passed from one nuraghe to the next may have served to broadcast news of the fall of Troy. If you can see one, you can generally spot four or five others. But as there is nothing in writing, their original function remains a mystery. All we know is that when they were reused, during the iron age (circa 10th century BC), it was for worshipping the moon.

Further research has focused on Su Mulinu, near Villanovafranca, 50km north of Cagliari, as part of the Great Tyrrhenian Itinerary, a Franco-Italian heritage trail. A dig has uncovered a large bastion, on a clover-leaf plan, dating from 1400BC. It bears the visible scars of a fire, which occurred in about 1000. A limestone altar, itself shaped like a nuraghe, stands in the ninth-century-BC sanctuary. It is decorated with a crescent moon, a symbol of the mother goddess. From the sludge that covered the structure the archaeologists extracted gold, silver, amber and rock crystal jewellery, as well as hundreds of terracotta oil lamps thought to be offerings to the light of the sun, celebrated at the summer solstice until the second century. These finds are on view in a nearby museum.

The question remains as to what fearful catastrophe, circa 1175BC, plunged Sardinia into a “dark age”. Some islanders took refuge on high ground, others fled to Etruria (now central Italy). In his Life of Romulus, written in the second century, Plutarch maintains that the Etruscans had colonised Sardinia.

Along the coast of Italy Etruscan burial grounds have yielded up countless bronze figurines, Sardinian ex-votos featuring soldiers, with horned helmets and round shields, and models of nuraghi. If a tidal wave did occur, it might explain the large Campidano plain, which cuts across the southern part of the island from Cagliari to the Phoenician port of Tharros, on the west coast. In the Old Testament Ezekiel writes: “What city is like Tyrus, like the destroyed in the midst of the sea? […] In the time when thou shalt be broken by the seas in the depths of the waters […] All the inhabitants of the isles shall be astonished at thee …”

Frau quotes an inscription in the mortuary temple of Ramesses III (1184-1153BC) at Medinet Habu, Upper Egypt. It tells of how foreigners from the north saw the earthquake. Then the waters engulfed their land, the sea god Nun having stirred and sent a huge wave to swallow up towns and villages. The foreigners were probably Sardinian mercenaries employed by the pharaoh. So was this just a mythical event or a real disaster? The issue attracted a large number of local people in June, who crammed into the chapel of Santa Anastasia in Sardara, even spilling over into the street outside, to listen to the scientists. The conference was illustrated by plenty of photographs. After listening open-mouthed for two hours solid the audience broke into a storm of applause worthy of the first night of an opera.

Until the 1980s no one was aware that tidal waves had occurred in the Mediterranean

Professor Tinti explained that until the 1980s no one was aware that tidal waves had occurred in the Mediterranean. But since 2004 scientists have identified 350 events of this type over a 2,500-year period. “The earthquake in Algeria in 2003, which killed 2,000 people, triggered a shockwave that reached the Balearics and Sardinia an hour later,” he said. “So what would have been required in our case?” he then asked. “We’re talking about a huge volume of water, some 500 metres high [the elevation up to which the nuraghi were affected]. Only a comet could do that, if the impact occurred very close to the coast and in a very specific direction,” he asserted. An event of this sort may have occurred near Cagliari, with the resulting wave devastating the plain of Campidano.

“One of the merits of the research carried out by Sergio Frau is to have shown that the nuraghe civilisation was one of the focal points of the ancient world, in terms of both geography and outlook,” says Azzedine Beschaouch, former head of the Unesco world heritage centre. “Now we need to give scientific, historical, cultural, political and emotional substance to a still mysterious past.”

“A falling comet strikes the sea at a speed of 20km a second,” Tinti adds. “It takes less than a second for the wave to propagate, with a four or fivefold increase in size.” He is convinced that his theory is right. It remains to be seen whether evidence of its impact can be found underwater, perhaps even fragments of the projectile.

To this day the people of Sardinia are wary of the coast. As the Sardinian singer Clara Murtas puts it: “The sea, we do not name it, we shun it.”

This article appeared in Guardian Weekly, which incorporates material from Le Monde