Underwater Demolition Teams (UDT) were a precursor to the current United States Navy SEALs. The first group of volunteers was selected from the Naval Construction Battalions (Seabees) in the spring of 1943. These volunteers were organized into special teams called "Navy Combat Demolition Units" (NCDUs). The units were tasked with reconnoitering and clearing beach obstacles for troops going ashore during amphibious landings, and evolved into Combat Swimmer Reconnaissance Units; this is a type of frogman operation. Their first issue breathing set was this type of rebreather designed by Christian J. Lambertsen.
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