Two days before the first ascent of Mt. Everest in 1953, Tom Bourdillon and Charles Evans climbed to within 90 m of the summit at unprecedented speeds. By breathing pure oxygen from a closed circuit, the pair were able to obtain an enormous physiological advantage. Unfortunately, due to a malfunction in Evans's circuit, the pair abandoned their attempt on the South Summit. For many who used the circuit in the 1930s and 1950s, the device proved too heavy, uncomfortable, and tiring for mountaineering. These factors, together with the wider ethical concerns of using supplemental oxygen at altitude, have meant that closed-circuit oxygen has been ignored for more than 50 years. In this article the authors will attempt to describe the history of this discarded circuit and the experience of those who utilized it.