Skin diving, or diving without the aid of special equipment, is a technique to investigate underwater environments and to gather shellfish or other items of commercial importance. In breath-hold diving, fins and faceplate are usually employed to facilitate the diver's activities. For longer periods of time, divers can function by using air pumped to them from the surface. This restricts the area that can be explored since movement of the diver is limited by the length of the air hose. Diving with scuba (self-contained underwater breathing apparatus) provides an almost limitless amount of freedom. With this equipment, the amount of time the diver can stay beneath the surface and the distance traveled are limited only by the volume of compressed air in the diver's tanks or aqua-lung.