Submarines are pretty cool bits of kit. They can offer researchers an unrivaled view of the ocean floor, help search out missing wrecks beneath the waves and can even ferry about the rich and famous, ...

For deep-diving mixed-gas divers, it's far worse. Routinely diving below 130 ft (40 m), the divers are always in pitch darkness, relying on work and helmet lights, and they are tethered to the surface ...