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 km (24 mi). Sombrero is 1.67 kilometres (1,830 yd) long north-south, and 0.38 km (420 yd) wide. The land area is 0.38 km2 (94 acres). Originally, when viewed from the sea, the island had the shape of a sombrero hat but guano mining operations have left the island with precipitous sides and a relatively flat top which is 12 m (39 ft) 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.
The lighthouse marks the Anegada Passage, which is the route from Europe into the Caribbean. The first lighthouse was erected in 1868. In 1960, Hurricane Donna damaged this lighthouse, with the result that a new lighthouse replaced it in 1962. The lighthouses were manned from 1868 to 2001. In that year, Trinity House donated and installed the current, automated tower, which is a 15 m (49 ft) round tower, painted white. Anguilla's Department of Fisheries and Marine Resources is responsible for the maintenance of the navigational aids.
As a result of the Treaty of Utrecht in 1714, Sombrero passed into the hands of the British. Captain Warwick Lake of Recruit marooned an impressed seaman, Robert Jeffrey, there on 13 December 1807. As it turned out, Jeffrey survived. A passing American vessel, the schooner Adams from Marblehead, Massachusetts, had rescued him. Still, a court-martial dismissed Lake from the Royal Navy.
In 1814, and again in 1825, a British geologist surveyed the island and found that it abounded in guano and reported this to the British government.
In 1856 the Americans claimed the island, and in a very short period of time quarried 100,000 tons of phosphate that served as fertilizer for the exhausted lands of the Southern States. Uniquely, an important insurrection occurred when West Indian black workers revolted against the “slavery proclivities” of a white American superintendent vis-à-vis wage-earning free men. Four of the 200 workers "fatally injured" Superintendent Snow and commandeered the island and company money and stores.
The British later intervened and demanded compensation from the United States for the occupation. The conflicting claims to the island were settled in Britain's favour in 1867.
Sombrero, lying in the route of shipping from England to South and Central America, lay in an area with many hazards and in 1848 the Admiralty was asked to install a light on it. On 30 June 1859, the Royal Mail Steam Packet Company's ship Paramatta was wrecked on her maiden voyage on Horseshoe Reef, which resulted in another request to the Admiralty. The lighthouse was then built and first exhibited its light on the evening of 1 January 1868. In 1871, the lease of the island was sold for £55,000 and then sold again for £110,000 to the New Sombrero Phosphate Company, which led to litigation in Erlanger v. New Sombrero Phosphate Co (1878) 3 App Cas 1218.
From the early 1870s until 1885, a Cornish mining engineer, Thomas Corfield, was Superintendent of Sombrero. His duties included organizing the conveying of the guano to a spot which was convenient for loading the lighters to take the guano to the ships lying off the island, overseeing the construction of derricks and engine houses, and arranging for the laying of the tram lines for the wagons, which were loaded at the quarries. The guano was just piled in dumps near the engine houses and derricks. There was no semblance of a port and no beach.
The black workers were recruited from various islands and lived in wooden huts during their term of service. Stores and various supplies were obtained from a merchant at Philipsburg, Sint Maarten, a Mr Nesbit. The company's schooner, the Logos, brought supplies and also took the black labourers to and from their homes on the other islands.
The superintendent's house was a wooden bungalow near the middle of the island and round it were grouped the quarters of the technicians, store keepers, and lighthouse keepers, and other wooden buildings. On the side opposite to the main buildings was a small building for the superintendent. There was a wide veranda round the house and he used to live there with his family, except near the hurricane season.
In 1890, the phosphate works on the island were abandoned and by 1893 the lighthouse had come under the authority of the British Board of Trade, later the Department of Transport. Administration of the light was carried out by Trinity House. In 1931, the old light system was changed and improved to 200,000 candle power and the tower received its first major repair as the basement was encased in concrete. On 20 July 1962, after the destruction caused by Hurricane Donna in 1960, the present lighthouse was put into operation and the old tower demolished on 28 July 1962. The lighthouse is located near the centre of the island, and reaches a height of almost 166 feet (51 m) above sea level. It protects ships passing from the Atlantic Ocean to the Caribbean Sea through the Anegada Passage. Full responsibility for the light passed from Trinity House to the Anguillan government on 1 December 2001.
Until recently the only inhabitants were the staff of the lighthouse but the light was automated in 2002 and now the island is uninhabited. The only visitors are the occasional fishermen, biologists engaged in fieldwork, and the occasional SCUBA group visiting the island for its interesting dive sites and post-apocalyptic surface.
The island is noted for the endemic Sombrero Ameiva, a widespread and easily seen lizard species on the island. A recently discovered dwarf gecko, Sphaerodactylus, may be endemic and has been tentatively named the Sombrero Dwarf Gecko. The Anguilla Bank Anole also inhabits the island.
- Masked Boobies Sula dactylatra: 27 pairs (54 + birds, 4% of Caribbean population) 2002
- Brown Boobies Sula leucogaster: 386 pairs (772 + birds, 5% of Caribbean population) 1999
- Bridled Terns Sterna anaethetus: 270 pairs (540 birds, 4% of Caribbean population) 1998
- Brown Noddies Anous stolidus: 700 pairs (1400 birds, 5% of Caribbean population) 1998
- Sooty Terns Onychoprion fuscata 
- New York Times, 13 August 1860: “Negro Insurrection on a Guano Island”.
- The Illustrated London News, Vol. 47, no. 1323, p. 17-18.
- Cases and Materials in Company Law. Oxford: Oxford University Press. 2008. p. 81.
- Notes by Thomas Joseph Corfield - Made July 1957.
- A Visit to Sombrero by P. Bannis, Government Information Service Bulletin Vol I, No 2, July 1978, which quoted from the writings of Mr. William Hodge in the Sombrero Visitors' Book
- Written Answers - House of Lords:Anguilla:Sombero Island Lighthouse (21 February 2005)
- Hansen, D. M.; Donlan, C. J.; Griffiths, C. J.; Campbell, K. J. (April 2010). "Ecological history and latent conservation potential: large and giant tortoises as a model for taxon substitutions". Ecography (Wiley) 33 (2): 272–284. doi:10.1111/j.1600-0587.2010.06305.x. Retrieved 2011-02-26.
- "Sombrero". BirdLife data zone: Important Bird Areas. BirdLife International. 2012. Retrieved 2012-11-23.
- UK Overseas Territories Conservation Forum. Ramsar Information Sheet: UK45006
- Lazell, J. (1964) "The reptiles of Sombrero, West Indies". Copeia: 716-718.