Saint Croix is an island in the Caribbean Sea, and a county and constituent district of the United States Virgin Islands (USVI), an unincorporated territory of the United States. It is the largest of the U.S. Virgin Islands, being 28 by 7 miles (45 by 11 km). However, the territory's capital, Charlotte Amalie, is located on Saint Thomas. St. Croix, like many other Caribbean islands, has tourism as one of its main sources of revenue. However, there are a number of other industries on the island to help support the economy. St. Croix was once an agricultural powerhouse in the Caribbean, but ended with the rapid industrialization of the island's economy in the 1960s. Buck Island Reef National Monument preserves a 176 acre (71 ha) island just north of Saint Croix and the surrounding reefs. This is a popular destination for snorkelers.