On 23 November 1943, the U.S. Marine landing and the subsequent battle at Tarawa Atoll emphasized the need for hydrographic reconnaissance and underwater demolition of obstacles prior to any amphibious landing. After Tarawa, 30 officers and 150 enlisted men were moved to the Waimānalo Amphibious Training Base to form the nucleus of a demolition training program. This group became Underwater Demolition Teams (UDT) One and Two.