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.

Hurd Deep running from bottom left to top right of an extract from a 1955 Admiralty Chart
1955 Admiralty Chart No 2649 showing Hurd Deep in the context of the English Channel