Fort De Soto was built to stop an enemy fleet from forcing the mouth of Tampa Bay, and for forty years that enemy never came. Then, in 1942, a seaborne enemy finally did reach these waters, and it came in a form the fort's designers could never have fired upon: German submarines, hunting the tankers and freighters that fed the American war effort, in the Gulf just off the bay's entrance. The closest the war ever came to Mullet Key was not a battleship on the horizon but a torpedo in the dark, and the marks it left are still on the seafloor today.
After Pearl Harbor, Admiral Karl Donitz unleashed his U-boats against the unprotected shipping of the American coast, an offensive the Germans called the “second happy time.” The U.S. Navy organized the Gulf Sea Frontier on 6 February 1942 to defend the southern coast from Jacksonville around to Texas, but it had almost nothing to defend it with, a handful of cutters, yachts, and aircraft against modern submarines. In early May 1942 Donitz extended the campaign into the Gulf of Mexico itself, and it became, for a few months, one of the deadliest stretches of water on earth. Over roughly five months, fourteen of the twenty-three U-boats sent into the Gulf sank fifty-six Allied ships, most of them oil tankers, the lifeblood of the war. American censors suppressed the news to prevent panic, a silence so effective that the Gulf U-boat war is still half-forgotten.
The campaign's opening days brought it almost to Tampa Bay's doorstep. Before dawn on 5 May 1942, the American tanker Joseph M. Cudahy was steaming up the Gulf when her crew saw another tanker, the Munger T. Ball, burning on the horizon, torpedoed minutes earlier by the U-boat U-507. The Cudahy's master turned and ran for the safety of Tampa Bay, zigzagging at speed. He did not make it. The same submarine put a torpedo into her starboard side, and she went up in flames; twenty-seven of her thirty-seven men died, and the few survivors were pulled from the water and carried to Key West. She was steering for this bay when she died. By one local account, U-boats sank seven ships bound to or from the Tampa Bay area during the war, and the connection ran the other way too: as the Tampa Bay History Center's historians note, damaged ships were towed into Tampa's repair yards and survivors of torpedoed vessels were brought ashore here to recover.
This is where Mullet Key rejoined the war. The island had been taken back by the military in 1941 as the Mullet Key Bombing and Gunnery Range, a sub-post of MacDill Field, and the crews who trained over its targets flew the same patrols that hunted the submarines: B-17s from MacDill ranged the Gulf and Caribbean on anti-submarine duty, and the Coast Guard air station at St. Petersburg flew floatplanes, OS2U Kingfishers each slung with a depth charge under each wing, over the bay and its approaches through 1942 and 1943. At the mouth of the bay, the narrow ship channel that Fort De Soto had been built to command was now swept for mines and watched by a guard vessel stationed off Egmont Key, and the coastal towns went dark under wartime blackouts, their streetlights doused and windows curtained so that no ship would be silhouetted for a waiting submarine. The fort's old mortars were useless against this enemy; the bay was defended instead by aircraft, blackouts, and the convoy system that finally, by the end of 1942, drove the U-boats off.
The war left a long underwater tail in these waters, and it is still being read. In 2010 Congress directed the National Oceanic and Atmospheric Administration to assess the thousands of sunken wartime wrecks that might still leak oil; the resulting survey, the Remediation of Underwater Legacy Environmental Threats, screened more than twenty thousand vessels down to eighty-seven priority wrecks, the torpedoed tankers of 1942 among them. The wreck long known to divers off this coast as the “Oil Wreck,” trailing a sheen for sixty years, was thought to be the Cudahy; when a Coast Guard, Navy, and NOAA team finally surveyed and remediated it in 2021, marine archaeology revealed it was actually her sister victim the Munger T. Ball, and a plaque was laid on the hull for the thirty men who died there. Deeper in the Gulf lies the enemy's own loss: U-166, the only German submarine sunk in the Gulf of Mexico, depth-charged in 1942 by the patrol craft escorting a ship it had just torpedoed, and lost to history until an oil-pipeline survey's robotic submersible found it on the seafloor in 2001 and confirmed the long-doubted account of its sinking. Closer to home, the underwater legacy is humbler and more hazardous: not every bomb and shell fired on the Mullet Key range exploded, and the Army Corps of Engineers still carries the island and its shallows as a Formerly Used Defense Site, where submerged ordnance from the war is surveyed and cleared so that swimmers are safe.
There is a deep irony in the U-boat war for this archive. For the only time in its history, the thing Fort De Soto existed to prevent, a hostile enemy attacking shipping at the mouth of Tampa Bay, actually came to pass. But it came half a century too late for the fort, and in a form its great mortars could not touch. The enemy was beneath the surface, and the answer was the airplane and the convoy, not the coastal gun. Mullet Key did serve in this war, as a place to practice, and the bay it guarded did finally see the enemy it had always waited for. The fort just had nothing left to say to him.