The island is about 48 kilometers west of Manila. It is shaped like a tadpole, with its tail running eastward, and has a land area of 9 km². Along with Caballo (which lies 2 km south of the "tail's" tip), it partially blocks the entrance to Manila Bay, and thus has strategic importance. It also creates a northern and southern entrance to the bay.
Because of its rocky landscape and the fortifications of Fort Mills, the island was also known as "the Rock"[cite this quote].
Geologically the island is a remnant of a volcano, Corregidor Caldera, last active about 1 million years ago. Philippine Institute of Volcanology and Seismology regards it as a potentially active volcano.
Corregidor is four miles (6 km) long and about 1.5 miles (2.4 km) at its widest point, with a total area roughly about three square miles. Its bulbous head, which points towards the South China Sea rises prominently to a large flat called Topside. This was the nerve center of the Island and here was located the headquarters, barracks for enlisted personnel, a branch of the Philippine Trust Co. bank, the Cine movie theater, officers' quarters, underground ordnance shops, the traditional parade grounds, an Officers' Club with a 9 hole Golf Course , tennis courts, and swimming pool, and the bulk of the batteries that constituted the strength of Corregidor.
