Between these, a few others, and the shores of Kent, Suffolk and Essex are many long shoals in the North Sea, broadly shallow enough to wreck vessels of substantial draft at low tide.
Its open sea end is distant from but due east of Foulness Point and due south of Clacton-on-Sea (both in Essex) and is bounded by two large typical sandbank form of shoals, the Knock John and Sunk Sands to the north-west, and the Girdler – and Long Sands, the largest of those away from the shore – spread out to the south and east.
Deep-draught vessels making for the Port of London from the North Sea use it to approach from the north-east from the position of the Sunk lightship, thence into the narrow Knock John channel, to enter the Tideway (lower Thames).
[1] Passing south of the West Knock buoy off Shoeburyness, large bulk carriers also tend to use the channel when entering or exiting the Medway estuary as it has a minimum depth of 14 metres as far as Kingsnorth.
[5] As of strategic importance, access to the Deep has been restricted in wartime, while during the Second World War naval forts stood at Sunk Head and Knock John to deter German minelaying.