High-altitude drop/low-altitude parachute opening (High Altitude-Low Opening). A parachuting technique in which the parachutist falls from an aircraft flying at a high altitude exceeding 10,000 feet (more than 3 kilometers) and free falls to an altitude of 1,000 feet (approximately 300 meters) from the surface before opening his chute. Because the parachutist falls from an aircraft cruising at such a high altitude that cannot be perceived from the ground, the risk of detection is greatly reduced. Also makes it possible to close in on the landing zone with high precision. This parachuting technique was developed by the SAS in the mid-1960s.

In 1964, FOX's infiltration agent, Naked Snake, performed a HALO jump from an altitude of 30,000 feet to successfully infiltrate Soviet territory. This is unofficially regarded as the world's first HALO drop.