Deoxygenated air poses a serious, life-threatening hazard (confined space hypoxia) for engineering projects in London, particularly within the Upnor Formation of the Lambeth Group. This paper reviews its causes and postulates that it was induced by regional-scale dewatering and drawdown of the Lower Aquifer during the industrial growth of London, bringing air into contact with these sediments, which became oxidized. Ensuing post-industrial recharge and resaturation resulted in accumulations of often compressed deoxygenated air, trapped beneath overlying impermeable clay strata. Historically, glauconite has been considered to be the mineral most likely to remove oxygen from the trapped air, but it lacks potency as a reducing agent and remains unaltered in many oxidized sediments. Pyrite and organic carbon are both plausible, but rare. Green rust, a mixed Fe(II) and Fe(III) layered double hydroxide, is more likely, as it rapidly oxidizes on contact with air and is only briefly observed in fresh core samples. Two key ground engineering hazards are, therefore, identified: the misidentification of risk by reliance on observations of glauconite; and the likelihood of encountering pressurized deoxygenated air within Lambeth Group sediments in underground projects following short- or long-term changes in the groundwater level.