Jack Sound is a treacherous body of water about 800 metres (2,624.7 ft) wide between the island of Skomer

Skomer

Skomer is a 2.92 km² island off the coast of southwest Wales, one of a chain lying within a kilometre off the Pembrokeshire coast and separated from the mainland by the treacherous waters of Jack Sound....

 and the Pembrokeshire

Pembrokeshire

Pembrokeshire is a county in the south west of Wales. It borders Carmarthenshire to the east and Ceredigion to the north east. The county town is Haverfordwest where Pembrokeshire County Council is headquartered....

 mainland that contains numerous reefs and a tidal race

Tidal race

Tidal race is a natural occurrence whereby a fast moving tide passes through a constriction resulting in the formation of waves, eddies and hazardous currents...

 of up to 6 knots.

It is used by boats to avoid a three-mile detour around the island, and has its share of sunken shipwrecks.

The most popular wreck for divers

Scuba diving

Scuba diving is a form of underwater diving in which a diver uses a scuba set to breathe underwater....

 is The Lucy, which sank in good condition in 1967 after being abandoned by its crew owing to its cargo of calcium carbide

Calcium carbide

thumb|right|Calcium carbide.Calcium carbide is a chemical compound with the chemical formula of CaC2. The pure material is colorless, however pieces of technical grade calcium carbide are grey or brown and consist of only 80-85% of CaC2 . Because of presence of PH3, NH3, and H2S it has a...

.

The island and the sound are both part of the Skomer Marine Nature Reserve

Skomer Marine Nature Reserve

Skomer Marine Nature Reserve is one of only two such sites in Britain. It is located off the coast of Pembrokeshire.The reserve completely surrounds the islands of Skomer and Middleholm and encompasses the mainland coastline around the end of the Marloes peninsula, including the small bay of...

.