The precursor to the present-day Navy SEALs, the Underwater Demolition Teams (UDT) were an elite special-purpose force established by the United States Navy during World War II. They also served during the Korean War and the Vietnam War. The UDTs reconnoitered beaches and the waters just offshore, locating reefs, rocks, and shoals that would interfere with landing craft. They also used explosives to demolish underwater obstacles planted by the enemy.

As the U.S. Navy's elite combat swimmers, they were employed to breach the cables and nets protecting enemy harbors, plant limpet mines on enemy ships, and locate and mark mines for clearing by minesweepers. They also were the frogmen who retrieved astronauts after splashdown in the Mercury through Apollo manned space flight programs. The UDTs pioneered combat swimming, closed-circuit diving, underwater demolitions, and midget submarine (dry and wet submersible) operations.