Helmet (Hard-Hat) Diving
Although early diving bells provided divers some protection and an air supply, they limited the diver's mobility. In the 17th and 18th centuries, a number of devices (usually made of leather) were developed to provide air to divers and to afford greater mobility. However, most of these devices were not successful because they relied on long tubes from the surface to provide air to the diver and thus did not deal fully with the problem of providing adequate quantities of compressed air.

The first major breakthrough in surface-supplied diving systems came in the early 19th century in the form of a helmet-and-suit apparatus that consisted of a rigid helmet sealed to a flexible waterproof suit. Pressurized air was pumped down from the surface into the helmet. This type of equipment, with a few refinements, is still in use today.

 In the 20th century, hard-hat divers learned that breathing mixed gases, in particular a helium-oxygen mixture, permitted them to dive to greater depths for longer periods than had been possible with regular air mixtures. Although surface-supplied diving has several advantages in terms of stability, air supply, and length of work period, major problems with hard-hat gear include severely limiting the diver's mobility, requiring support personnel on the surface, and cost. Scuba gear frees the diver from surface support and enables extraordinary mobility never before achieved in diving.