Across the entrance to Manila Bay are several islands, the largest of which is Corregidor, located 3 kilometres from Bataan and, along with the island of Caballo, separates the mouth of the bay into the North and South Channels. In the south channel is El Fraile Island and outside the entrance, and to the south, is Carabao Island. El Fraile, a rocky island some 4 acres (1.6 ha) in area, supports the massive concrete and steel ruins of Fort Drum, an island fortress constructed by the United States Army to defend the southern entrance of the bay. To the immediate north and south are additional harbors, upon which both local and international ports are situated. Large number of ships at the North and South harbors facilitate maritime activities in the bay. Being smaller of the two harbors, the North Harbor is used for inter-island shipping while the South Harbor is used for large ocean-going vessels.