Sicily’s hills were 40 metres below water during Earth’s megaflood

Jumbled deposits of rock found on the top of hills in south-east Sicily were left by the megaflood that refilled the Mediterranean sea 5 million years – the largest known flooding event in Earth’s history.

The rock deposits and eroded hills in this part of Sicily, a region of Italy, are the first land-based evidence found for the megaflood, says Paul Carling at the University of Southampton in the UK. “You can actually walk around and see it,” says Carling.

Around 6 million years ago, during the so-called Messinian salinity crisis, the Mediterranean Sea was cut off from the Atlantic Ocean and began to dry out. Vast deposits of salt formed at this time and the sea level may have dropped by a kilometre or more.

Water once again started flowing through the Strait of Gibraltar into the Mediterranean around 5.3 million years ago. Researchers initially thought an enormous waterfall near Gibraltar refilled it over a period of tens of thousands of years.

But in 2009, the discovery of a massive eroded channel on the bottom of the strait pointed to a much more abrupt megaflood. The evidence for this has been growing ever since.

This megaflood first filled up the western basin of the Mediterranean Sea, says Carling. Eroded features on the seafloor suggest it then spilled over the underwater ridge, known as the Sicilian sill, into the eastern basin.

Team member Giovanni Barreca at the University of Catania in Italy, who grew up in south-east Sicily, suspected the land there was also shaped by the megaflood. So he and his fellow researchers took a closer look and analysed rock samples.

Sure enough, they found the jumbled deposits near the top of some hills contain rocks that have been eroded from much deeper layers and somehow carried up to the top of the hill. “You can tell from their nature that they were from these lower levels,” says Carling. “And they were carried up and over these hills.”

Many of the hills themselves have a streamlined shape, and resemble ones in Montana that were sculpted by a massive flood caused by an ice dam breaking at the end of the last glacial period. “They’re quite distinctive,” says Carling. “And the only thing can streamline features of this scale is very large-scale, deep flooding.”

The team estimated during the peak of the flood the water was flowing at around 115 kilometres per hour and covered the tops of the hills – which are around 100 metres above the modern-day sea level – with about 40 metres of water.

The researchers also studied the seafloor around Sicily and found yet more evidence for the megaflood, such as eroded ridges and channels. Their modelling suggested the entire Mediterranean Sea refilled in between two and 16 years, but the main flooding event in Sicily probably lasted only days, Carling says.