Sombrero, also known as Hat Island, is the northernmost island of the Lesser Antilles in position 18° 60'N, 63° 40'W. It lies 34 miles (55 km) north west of Anguilla across the Dog and Prickly Pear Passage. The distance to Dog Island, the closest island of Anguilla, is 24 miles (39 km). Sombrero is 0.9 miles (1.4 km) long north-south, and 0.25 miles (0.40 km) wide. The land area is 95 acres (380,000 m2). Originally, when viewed from the sea, the island had the shape of a sombrero hat but mining operations have left the island with precipitous sides and a relatively flat top which is 40 feet (12 m) above sea level. The surface of the island is rough, and vegetation is sparse.
This mining operation yielded some 3000 tons of phosphate a year by 1870. By 1890, the phosphate reserves had been exhausted.