The Site is formed by Sombrero Island, an offshore cay, and the surrounding sea; it lies some 65 kilometres north-west of the mainland of Anguilla. The Island is a remote, flat-topped rocky outcrop, characterized by its karst topography. The sparse vegetation is concentrated on the eastern half. The otherwise-flat Island was once mined for phosphate and is now marked by craters up to ten metres deep. These craters, and the shallower pits formed over time through natural weathering and erosion (particularly along the shoreline) often form shallow pools in which periwinkles and small reef fish can be found, attracting resting and feeding seabirds. The Island was significantly damaged by Hurricane Luis in 1995, but the vegetation is recovering and once more provides important habitat to the endemic Sombrero Island ground lizard Ameiva corvina. Over 40 endemic species of insect have been documented. The Island was listed in 2005 as an Important Bird Area for its nesting population of bridled tern Sterna anaethetus. Brown booby Sula leucogaster, masked booby Sula dactylatra and brown noddy Anous stolidus also nest there in internationally important numbers.