Volcanic and coral archipelago, southwestern Philippines, between Mindanao and Borneo .
A double island chain, it extends 170 mi (270 km) and includes about 400 named islands and more than 500 unnamed ones; they cover an area of 1,038 sq mi (2,688 sq km). The islanders were converted to Islam by Abu Bakr in the mid-15th century. The Spanish tried, at first unsuccessfully, to subdue the inhabitants, whom they called Moros . The islands finally became a Spanish protectorate in the 19th century, and in 1899 came under U.S. authority. The archipelago was ceded to the Philippines in 1940. The islands provide a haven for smugglers and pirates.