“Transfer to the chamber for blowdown,” echoed a canned voice from a speaker mounted on the wall. It was Dive Control, the command center for all diving activities on the Depth Charge, housed in a dark room in the ship’s belly. The divers each lifted a gloved hand to the camera mounted in the corner of the room and gave the “OK” sign with index finger and thumb pressed together.

Aitkens and McElroy were saturation divers, about as far from recreational scuba divers as a kite-flying child is from a fighter pilot. Rather than swimming freely with tanks on their backs, their breathing gas, a mix of helium and oxygen, would be continually pumped down to them through a long “umbilical” system from the support ship. This umbilical also bundled communication cables and electrical power for their headlamps and camera. A hot water system circulated warm water through their suits. Though the surface temperature of the Indian Ocean was 87 degrees Fahrenheit, at 350 feet, it would be in the 60s, enough to chill a diver working for several hours.

The divers would not travel to and from the sea bottom by their own power, but rather be transported inside a diving bell, pumped up to the same pressure as the surrounding water at depth, thus staying dry inside. Once at the bottom, the divers exited the bell and swam or walked to their work sites, where they could stay for hours. To get around lengthy decompression, the men would remain under pressure inside the bell on the way back to the surface, and then be transferred to a pressurized chamber on board the Depth Charge, where they could either remain at “bottom pressure” or slowly decompress for hours or even days while the ship sailed off-site.

This kind of “saturation” diving has been around since the 1960s, and helped pioneer much of the North Sea and Gulf of Mexico oil fields, as well as numerous clandestine military operations carried out during the Cold War. But this empty stretch of Indian Ocean off the east coast of Sri Lanka held no oil reserves, nor was this a military operation.

Murray, McElroy, and Aitkens ducked into the compression chamber, a schoolbus-length tube of thick steel in the hold of the ship. This would be their home between dives and during the long decompression to bring them slowly back to surface air pressure. An attendant sealed the hatch behind them and checked for leaks. Then the chamber was “blown down,” slowly pressurized to ten times the air pressure at the surface. The divers inside wiggled their jaws and blew against pinched noses as the atmosphere inside pushed against their eardrums.

A face appeared in the thick, small window halfway up the side of the chamber. It was Malcolm Rausing. He pressed a button next to the window and spoke quietly into a microphone.