Hurd's Deep

It's believed that the collapse of a chalk ridge that once dammed the Strait of Dover let flood waters from a huge proglacial lake flow through the former river systems scouring down to the bedrock forming the trench.

[2][3] During the Last Glacial Period, which ended 11,700 years ago, sea levels dropped again to the point that the English Channel became an area of river valleys.

[4] Hurd's Deep has an approximate length of 150 kilometres (80 nautical miles) with a width of between 2 and 5 km (1 and 2+1⁄2 nmi).

[6] Following the First World War, Hurd's Deep was used by the British Government as a dumping ground for both chemical and conventional munitions.

[7] Following the Second World War, it was used to dump military equipment, munitions and weaponry left behind by the ousted German invaders of the Channel Islands.