Underwater diving is the practice of descending below the water's surface to conduct underwater activities. In ambient pressure diving, the diver is exposed to the pressure of the surrounding water, and uses breathing apparatus such as that used for scuba diving or surface supplied diving, or when freediving, will breath-hold for the duration of the dive. The saturation diving technique can be used to reduce the risk of decompression sickness after long duration deep dives. Atmospheric diving suits may be used to isolate the diver from the effects of high ambient pressure. Although not usually considered to be diving, the use of crewed submersibles can extend the range of possible diving depths, and remotely controlled and robotic diving machines can be used where the risks to human divers