Sombrero, also known as Hat Island, is part of the British Overseas Territory of Anguilla and is the northernmost island of the Lesser Antilles. It lies 54 km (34 mi) north-west of Anguilla across the Dog and Prickly Pear Passage. The distance to Dog Island, the next nearest island of Anguilla, is 38 kilometres (24 miles). The guano-mining operation yielded some 3000 tons of phosphate a year by 1870. By 1890, the phosphate reserves had been exhausted.