The diving bell rests on a principle Aristotle recorded in the 4th century BC: an upturned cauldron forced straight down traps its air. In 1535 Guglielmo de Lorena used a bell to explore Roman barges in Lake Nemi, staying down over an hour until cold and exhaustion drove him up. The 1600s brought an underwater gold rush: Edward Bendall cleared a wreck from Boston Harbor in 1642, Albrecht von Treileben salvaged bronze cannons from the Vasa in 1658, and William Phips recovered jewels, gold, and about 30 tons of silver from a Spanish galleon off Haiti in 1687. Carbon dioxide was the enemy, and in 1691 Edmund Halley, of comet fame, lowered lead lined barrels of fresh air whose water pressure pushed air through a leather hose into his bell.

In 1783 Edinburgh confectioner Charles Spalding and his nephew suffocated in his improved bell off Dublin. Modern wet bells stay open at the bottom and are fed gas from the surface, and divers ascending in them must decompress for hours. Closed bells sealed the problem, carrying saturation divers between pressurized deck chambers and the seabed for weeks of work. Submarine rescue bells reverse the idea, clamping onto a sunken sub’s hatch so sailors ride up at surface pressure, while airlock bells like the British Admiralty’s 1902 Gibraltar bell let workers walk down a tube in street clothes.

• A Type 1 wet bell runs divers’ umbilicals to the surface, while a Type 2 feeds them from a panel inside the bell.
• A closed bell’s bottom hatch opens inward, so internal pressure presses it tighter against its seals.
• Divers pinned to the bottom can cut their own umbilical from inside and float up on emergency gas and soda lime scrubbers.
• Launch systems guide bells down tensioned clump weight cables, and aerated moon pools soften their entry into the water.
• The diving bell spider, Argyroneta aquatica, stocks an underwater web dome with air carried in its body hairs.