Modern helmet diving suits usually consist of a waterproof one-piece suit made of canvas and rubber that entirely covers the wearer except for the head and hands. Heavy rubber bands seal the suit at the wrists, leaving the hands free. On the feet the diver wears leaded boots weighing about 40 lb (18 kg), and lead weights are fastened to the chest to maintain equilibrium. A metal helmet with side and front windows covers the head. A noncollapsible pipe connects the helmet to an air supply. An attached lifeline hauls the diver to the surface. Too rapid an ascent from great depths causes the diver to suffer decompression sickness. To prevent this, deep-sea divers either use an all-steel, armored diving suit or breathe a special mixture of nine gases developed by the Swiss mathematician Hannes Keller.
Helmet diving has the disadvantage of restricting the diver's lateral movement because of the connection to the surface. This fact led to the development of scuba (an acronym for self-contained underwater breathing apparatus). Scuba delivers air to the diver (from tanks of compressed air) at the same pressure as that exerted by the surrounding water. In this way the diver is able to descend to great depths without feeling the ill effects of high pressure (see skin diving). A skilled scuba diver with good equipment can descend as deeply as a helmet suit diver.
Record-setting dives of over 300 ft (91 m) have been made with scuba gear, although careful scuba divers do not go below about 130 ft (40 m). Beyond this depth a condition known as nitrogen narcosis (popularly called "raptures of the deep") tends to set in. Caused by the narcotic effects of the air's nitrogen at high pressure, the condition is marked by a loss of judgment that often causes the diver to discard equipment or engage in other dangerously foolish behavior. Nitrogen narcosis also affects helmet suit divers, but not until a depth of about 200 ft (61 m).