The idea was audacious. After the Spanish-American War, the Board of Fortifications -- chaired by William H. Taft himself -- recommended defending Manila Bay's wide South Channel entrance. El Fraile Island, a craggy rock where Spanish guns had fired at Commodore George Dewey's squadron during the 1898 Battle of Manila Bay, was chosen for transformation. Army engineers didn't simply build on the island. They leveled it entirely, then constructed a massive concrete hull on top, complete with 20-foot-thick reinforced concrete overhead protection and exterior walls up to 25 feet thick. The original plan called for 12-inch guns, but the War Department upgraded them to four 14-inch guns in twin armored turrets -- weapons specially designed for Fort Drum and deployed nowhere else. Secondary armament included four 6-inch guns in armored casemates along the sides, exactly as on a real battleship. Construction took five years. When it was finished, approximately 240 officers and enlisted men lived deep inside the structure, surrounded by power generators, plotting rooms, and ammunition magazines.

When Japan invaded Luzon in late December 1941, Fort Drum's garrison of the 59th Coast Artillery Regiment tore down the wooden barracks on deck to clear fields of fire. On January 13, 1942, a newly installed 3-inch gun at the fort's stern -- its concrete emplacement not yet fully dry, the barrel not even bore-sighted -- became the first American seacoast artillery piece to fire on the enemy in World War II, driving off a Japanese steamer probing the fort's vulnerable rear. What followed was months of punishment. Japanese 150mm howitzers opened up from the Cavite mainland in February, and by March, 240mm siege howitzers had joined in, destroying the antiaircraft battery and disabling one 6-inch gun. Sizeable chunks of concrete were chipped away. But the armored turrets held. The 14-inch guns never went out of action. On the night of May 5, as Japanese forces stormed Corregidor, Fort Drum's batteries sank several troop barges and inflicted heavy casualties. The fort surrendered the next day -- not because it was beaten, but because Corregidor had fallen. Not a single American soldier inside Fort Drum was killed during the entire siege. Only five were wounded.

Three years of Japanese occupation ended in April 1945 with a method as brutal as it was effective. Company F of the 151st Infantry Regiment had already perfected the technique at nearby Fort Hughes on Caballo Island: pump fuel through a vent, then ignite it. A modified Landing Ship Medium pulled alongside Fort Drum with a bridging arm, and American troops sprinted directly onto the top deck, confining the 68-man Japanese garrison below. Then came 2,500 gallons of diesel and gasoline, poured through the ventilation system. White phosphorus mortar rounds ignited the mixture. The explosion blew a one-ton hatch 300 feet into the air and shattered sections of the reinforced concrete walls. Internal fires raged for days. Five days passed before the heat subsided enough for anyone to enter. Six Japanese soldiers had suffocated on the upper floors; the charred remains of the other 62 were found in the boiler room. Fort Drum never served again.

Today Fort Drum sits abandoned in Manila Bay, its concrete hull slowly weathering but still recognizable from the air as something that doesn't belong in open water -- a ship that is not a ship, built on an island that is no longer an island. The Philippine Coast Guard maintains a light on the top deck to guide vessels through the South Channel. The fort's story has seeped into fiction: in Neal Stephenson's novel Cryptonomicon, a character parachutes onto its deck during the 1945 recapture, and a Call of Duty video game features it as a multiplayer map. But the real Fort Drum needs no embellishment. It remains the only American sea fort ever built with battleship turrets -- a structure so overengineered that Japanese siege artillery could chip its walls but never silence its guns, and so thoroughly destroyed in recapture that its concrete shell became a crematorium. The island under the jurisdiction of Cavite City endures as one of the Pacific War's most extraordinary artifacts, visible proof that someone once looked at a rock in the ocean and decided to turn it into an unsinkable warship.