UDT were an elite special-purpose force established by the US Navy during Word War II. Their primary function was to destroy enemy defensive obstacles on beaches where U.S. and allied forces were going to make amphibious landings.
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 also 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 conducted river surveys and foreign military training.
The UDTs also served during the Korean War and the Vietnam War.
The UDTs pioneered combat swimming, closed circuit diving, underwater demolitions, and midget submarine operations.
They were the precursor to the present-day United States Navy SEALs.