The standard diving dress, the classic copper hat rig, could weigh up to 190 pounds of canvas, rubber, lead, and spun copper, and it began as firefighting gear. In 1820s England, John Deane watched horses trapped in a burning stable and in 1823 patented a smoke helmet fed by a hose and a double bellows. Lacking capital, the Deane brothers sold the patent to their employer Edward Barnard, who brought in engineer Augustus Siebe. By 1828 the helmet had gone underwater, but the suit was open at the bottom like an upturned glass, so a diver who tripped or leaned too far could flood and drown. Konrad Kyeser had sketched a leather diving jacket and helmet in 1405, but only 19th century materials such as Charles Macintosh’s waterproof canvas and vulcanized rubber made a working suit possible. In the 1830s Siebe sealed the helmet to a watertight suit with an exhaust valve, and in 1834 Charles Deane walked the wreck of HMS Royal George and recovered 28 bronze cannons.

The episode breaks down the gear: the bonnet and corselet locked by a one eighth turn interrupted thread, shoes of up to 34 pounds, lead weights, and the US Navy Mark V belt of over 80 pounds. Surface crews hand cranked air pumps and read depth from the delivery pressure gauge, since every 33 feet adds an atmosphere. If a hose parted with no safety valve, the suit instantly lost pressure and the sea forced the diver’s body up into the rigid helmet, a fatal helmet squeeze prevented only by the non-return valve. The opposite danger was the blow-up, when air trapped in the legs flipped a diver and rocketed him upward, risking the bends, torn lungs, and a burst suit that sent him sinking again.

• In 1874 Auguste Denayrouze built a speaking tube with a diaphragm at each end, later replaced by electrical diver telephones.
• Tenders used soapy water or metal cuff expanders to force a diver’s hands through tight rubber wrist seals.
• Divers pressed a valve button with their chin to dump air and control buoyancy.
• A spitcock let a diver sip seawater and spit it onto the viewports to clear fog.
• Communication once relied entirely on tugs on the lifeline.