The Waage Drill II diving accident occurred on 9 September 1975, when two divers died of heatstroke after the chamber they were in was inadvertently pressurised with helium gas. Background On 9 September 1975, divers Peter Holmes, 29, and Roger Baldwin, 24, had been hoisted from the North Sea in a bell and connected to the system's entrance lock. The men had just completed a short dive to to clear a tangle of rope that had wrapped itself around the guideposts of the blowout preventer. The dive had gone well and now the plan was to decompress the men inside the bell to , then transfer them into the chamber complex and hold them in saturation. As with all deep-dive systems, each chamber on the supervisor's control panel was represented by a series of valves and gauges. Redundancy in plumbing schemes was common and necessary, and with this particular system, by turning several valves on the console, any one depth gauge could be made to monitor the depth of a chamber other than for which it was normally intended. On chamber one's panel, there was a Heise gauge considered to be the most accurate. Because of the cross-referencing capabilities of the system, it became the practice of the shift supervisor to set the valves of this gauge to read the internal depth of the bell prior to the divers leaving bottom, then track their ascent through the lock‑on and transfer procedure. The rationale behind using this particular gauge throughout the operation was to avoid any potential decompression problems that might arise from using two separate gauges with a discrepancy problem. Once the divers had safely passed from the bell to the entrance lock to chamber one, the supervisor was then supposed to turn the valves back to their original positions in order to monitor the depth of the divers. Accident At 21:50 that evening, the crew mated the bell to the entrance lock as planned, but during the lock‑on procedure a gas leak developed between the mating flanges. The bell was removed, the flange surfaces were cleaned, and on the second attempt the bell was successfully sealed to the system. After Holmes and Baldwin equalized the bell with the rest of the complex, they opened the inside door and were in the process of transferring into the entrance lock when the gas leak suddenly returned. With the needle on the