Unerwater Demolition Teams (UDT) were a precursor to the current United States Navy SEALs. The UDT's conducted beach and hydro-reconnaissance, explosive cable and net cutting; explosive destruction of underwater obstacles to enable major amphibious landings; limpet mine attacks, submarine operations, and the locating and marking of mines for minesweepers. They also conducted river surveys and foreign military training. While doing this, the SEALs’ predecessors pioneered combat swimming, closed-circuit diving, underwater demolitions, and mini-submarine (dry and wet submersible) operations.