The reef areas to the east of Mingulay in the Outer Hebrides are found within a wide trench in the seabed at depths of about 100 to 250 metres. Nine reef areas have been identified, formed by characteristic mounds on the seabed up to 150 metres high. An area of approximately 26 square kilometres supports reef habitat, including both biogenic and non-biogenic (rocky) reefs. The biogenic reefs, covering an area of about 5.4 square kilometres, are formed of the cold-water coral, Lophelia pertusa. Although L. pertusa is the main reef-forming coral in the northeast Atlantic and as a species is thought to be widespread, it rarely forms reef complexes in inshore waters (within 12 nautical miles of the coast). In fact, East Mingulay is unique in that it is currently the only known area with extensive cold-water coral reefs within UK territorial waters. Most of the East Mingulay Lophelia pertusa reefs form typical biogenic masses that host a large variety of associated species. The remaining reef areas within the site boundary include a rich mix of habitats and species developed on dead coral, boulders, and rocky and cobbly reef structures. Surveys of the East Mingulay reef complex have identified over 400 species, including a sponge Cliona caledoniae which was new to science (Van Soest and Beglinger, 2008).