An inlet of the Moray Firth in Highland Council Area, the Cromarty Firth separates the mainland of Easter Ross from the Black Isle. It extends 18 miles (30 km) west and southwest to Dingwall from its mouth between the headlands known as the Sutors of Cromarty. Invergordon was an important naval harbour during both World Wars until it closed in 1956. The broad, sheltered and deep waters between Nigg Bay and Cromarty Bay provide a safe anchorage but were the site of a disaster in 1915, when the cruiser HMS Natal blew up at anchor and sank with the loss of 405 lives. The cause was determined to be the accidental explosion of unstable cordite. Various attempts were made to salvage the wreck between 1926 and 1978, but efforts were concentrated on preventing a hazard to shipping. In 1972 a facility for the construction and repair of North Sea oil rigs opened in Nigg Bay.