island, Puerto Rico, 20 miles (30 km) east of Puerto Rico island and 15 miles west of St. Thomas, Virgin Islands. The island fronts north on the Atlantic Ocean and south and west on Vieques Sound, which connects the Atlantic with the Caribbean Sea. About 7 miles (11 km) long and 2 miles (3 km) wide, Culebra Island is 10 square miles (26 square km) in area. Its hilly, almost barren mass of limestone and igneous intrusions rises to 646 feet (197 metres) at Mount Resaca. The island's deep bay, Puerto Grande, on the southeast, was used as a U.S. naval base until 1975. Culebra has sparse, thin soils and no permanent streams; tourism and fishing are the principal activities of its few inhabitants.