HALO (High altitude, low opening) jumps are parachute drops where the plane flies at an extremely high altitude, and the jumper waits until he is very close to the ground before opening his parachute. This style of parachute jump is designed to protect both the plane and the jumper in hostile territory. The plane flies above any anti-aircraft weapons, and the jumper spends very little time floating underneath his canopy.
HALO jumps are effective in military situations, but they pose risks to the jumper. The extremely high altitude can cause hypoxia and decompression sickness if the parachutist is not properly prepared with oxygen before the jump. In addition, the late opening of the parachute gives very little time to switch to a backup chute if something goes wrong with the main canopy. Jumpers may also be exposed to temperatures low enough to cause frostbite.
The first military unit to employ HALO jumping was the Military Assistance Command, Vietnam - Studies and Observations Group. This unit used the technique to perform stealth drops into Laos during the Vietnam War. More recently, Navy SEAL teams have used the technique for special operations. SEAL Team Six developed the use of HALO to deliver equipment and boats along with its operatives.