 With an area of 118 square miles (305.7 square km), the group lies north of Nias and 18 miles (29 km) west of Sumatra in the Indian Ocean.The islands have long been noted for the existence of substantial offshore coral reefs, though problems with overharvesting, damage from explosives, and recent geological disturbances have threatened these underwater resources.The islands vary in their land cover; most are sandy with limited vegetation, while the larger islands have deep rainforests and are fringed by intertidal mangroves.