Beaufort Dyke is the sea trench between Northern Ireland and Scotland within the North Channel. The dyke is 50 km long, 3.5 km wide and 200–300 metres deep.
Because of its depth and its proximity to the Cairnryan military port, it became the United Kingdom's largest offshore dump site for conventional and chemical munitions after the Second World War; in July 1945, 14,500 tons of 5-inch artillery rockets filled with phosgene were dumped in Beaufort's Dyke. The Ministry of Defence estimates that there is well over a million tons of munitions at the bottom of Beaufort's Dyke. Munitions have subsequently been washed up on beaches in the area. In particular, in 1995, incendiary devices were discovered on the Scottish and Northern Irish coasts. This coincided with the laying of the Scotland-Northern Ireland pipeline (SNIP), a 24-inch gas interconnector being constructed by British Gas.
According to documents discovered at the Public Record Office approximately two tonnes of concrete-encased metal drums, filled with radioactive laboratory rubbish and luminous paint, were dumped into Beaufort Dyke, 300 metres deep and 10 kilometres off the Scottish coast.