Deep-sea diving is the descent by divers to the sea bed, usually for protracted periods, for purposes such as exploration and salvage. Skin diving is almost as old as man and the Romans had primitive diving suits connected by an air pipe to the surface. This principle was also known in the early sixteenth century. A breakthrough came when John Lethbridge devised the forerunner of the armored suits used today in deepest waters (1715): it looked much like a barrel with sleeves and a viewport, and was useless for depths of more than a few meters. In 1802 William Fowler devised a suit in which air was pumped to the diver by bellows. And in 1837 (improving his earlier design of 1819) Augustus Siebe (1788–1872) invented thefore runner of the modern diving suit – a continuous airtight suit to which air is supplied by a pump.
The diving suit today has a metal or fiberglass helmet with viewports and inhalation and exhalation valves, joined by an airtight seal to a metal chestpiece, itself joined to a flexible watertight covering of rubber and canvas; and weights, especially weighted boots, for stability and to prevent the diver shooting toward the surface. Air or, more often, an oxygen/helium mixture is conveyed to the diver via a thick rubber tube. In addition, he has either a telephone wire, or simply a cord which he can tug, for communication with the surface.
Nowadays SCUBA diving, where the diver has no suit but carries gas cylinders and an aqualung, is preferred in most cases since it permits greater mobility. In all diving great care must be taken to avoid the bends, or aeroembolism, caused by ascending too rapidly to the surface.
A diving bell is a hollow structure providing a dry environment for underwater workers. Early diving bells were bell-shaped, filled with compressed air, and open at the bottom to give access to the seabed. The bathysphere, which made its first dive in 1930, was spherical, made of steel, and could withstand considerable pressures, but it was restricted to depths within reach of supply and winch cables. It has been replaced by the bathyscaphe.