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An open-circuit system is the typical breathing system, where the diver breathes air from a supply tank and exhales exhaust into the water.
Closed-circuit Oxygen Systems
With this type of system, the diver breaths 100-percent oxygen, and his exhaled breath is recirculated within the apparatus, where it is filtered and turned back into breathable air. This system is useful for working in shallow water.
Oxygen time is reduced as the water gets colder. For diving in extremely cold water, SEALs must wear dry suits and a specially adapted version of the Dragger LarV rebreather — a larger oxygen canister allows the diver to breathe underwater for a longer period of time.
Closed-circuit Mixed Gas System
This system is similar to the closed-circuit oxygen system described above, but the oxygen is mixed with air to maintain a certain “partial pressure of oxygen” (PPO2) level. This increases the depth to which a SEAL can dive and the length of time he can stay there.
When SEALs arrive from the air, they are often going to extremely difficult-to-reach places. In this case, they may jump from a plane into the ocean with their Zodiac, parachute into the area, or use fast-rope and rappelling techniques.
When parachuting, SEALs use either static-line or free-fall techniques. Free-fall techniques include High Altitude/Low Opening (HALO) jumps and the more difficult High Altitude/High Opening (HAHO) jumps (see Navy SEALs.com: Air Equipment to learn about these types of jumps). High-altitude jumping requires oxygen and special equipment to ensure that the chute opens in the event the jumper blacks out, which is not that uncommon for high-altitude jumps. Goggles can shatter from the cold, and eyes can freeze shut, making the fall even more interesting. A device called an FF2 will automatically activate the jumper’s rip cord if the chute hasn’t opened at a preset altitude.