In September 1991, eight people entered Biosphere 2, a sealed glass structure in the Arizona desert designed to mimic Earth’s biosphere. Their mission was to live for two years using only the air, water, and food produced inside. But within months, oxygen levels began dropping unexpectedly, falling from 21% to around 14%—equivalent to the thin air at 4,000 meters elevation. Crew members reported fatigue and breathlessness, raising alarms.

Scientists initially expected oxygen loss to match carbon dioxide gain, but CO₂ levels didn’t rise as predicted. The answer emerged from two overlooked factors: microbes in the nutrient-rich soil consumed oxygen rapidly, and the structure’s uncured concrete absorbed carbon dioxide, converting it into calcium carbonate. This chemical process effectively removed CO₂ from the air, hiding the full picture.

Though the project was criticized for its scientific rigor and internal conflicts, the oxygen mystery provided a real-world lesson in closed ecological systems. It revealed how even inert materials like concrete can play a role in atmospheric chemistry when sealed at scale. The experiment ultimately showed how difficult it is to predict all interactions in a self-contained environment.