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Until now, the process by which a million cubic kilometres of salt accumulated in the Mediterranean basin over such a short period of time remained unknown.

Once the gravel is in place, the builders will add about 6,000 cubic yards of “engineered soil” to the structure, Rock said.

When it burst, it unleashed tens of millions of cubic metres of toxic waste and mud.

The researchers eventually worked out that 25 million cubic metres of rock - a volume equivalent of 25 Empire State Buildings - slammed into the water, causing a 200m-high “mega-tsunami”.

Crews hired by the contractor Kiewit Corp. have excavated an estimated 1 million cubic yards of rock, soil and clay at Iron Gate Dam.