Five thousand fathoms under the waves, a deafening clang rang out through the cramped, freezing submarine, causing the whole ­vessel to shake like a leaf.

Squinting through their tiny Plexiglas window into the abyss, the two explorers’ hearts missed
a beat.

‘It was a pretty hairy experience,’ they said afterwards with some understatement. The outer
layer of their porthole had cracked under the ­unimaginable weight of six miles of seawater — and they still had more than a mile to descend.