This tadpole-shaped island, 6.5-kilometer long and 2.5-kilometer wide, is the first thing that visitors see upon entering Manila Bay. Because of its strategic location in the entrance of the bay, it served as a fortress of defense from attacks by enemy warships for many centuries.
Geologically, the islands of Corregidor and Caballo are remnants of a volcanic crater that was last active more than one million years ago. The two islands are still classified by the Philippine Institute of Volcanology and Seismology as a potentially active volcano.