Air assault is the movement of ground-based military forces by vertical takeoff and landing (VTOL) aircraft—such as the helicopter—to seize and hold key terrain which has not been fully secured, and to directly engage enemy forces. In addition to regular infantry training, air-assault units usually receive training in rappelling and air transportation, and their equipment is sometimes designed or field-modified to allow better transportation within aircraft. In 1946, U.S. Marine General Roy S Geiger observed the atomic bomb tests at Bikini Atoll and instantly recognized that atomic bombs could render amphibious landings difficult because of the dense concentrations of troops, ships and material at beachheads. During this time, it was recommended that the USMC develop transport helicopters in order to allow a diffused attack on enemy shores. It also recommended that the USMC form an experimental helicopter squadron HMX-1.