Much of the oil has been removed and processed for use, but each time a large amount of oil is taken from the quarry, more seeps up from the sediment below and so the process has to start again.

BBC: Torrey Canyon seabed returns to normal after oil spill

That there are not could be explained by the fact that the accumulation of sediment in most rocks is so slow that the two layers are, in effect, superimposed.

ECONOMIST: Mass extinctions

Nautilus says that it is devising strategies for minimising the environmental impact, by trying to contain any disturbed sediment and leaving parts of the seabed untouched so the mined area can be recolonised by marine life.

BBC: UK Seabed Resources joins deep-ocean mineral-mining rush

"These were found in a shallow marine environment, so this beast must have been walking around, keeled over, died and the sediment must have gone over them and protected them for 12, 000 years, " Mr Mount said.

BBC: The tusks