The BIOS-3 facility in Siberia was a sealed 315m3 habitat used in the 1970s to research exactly this. The system used tanks of Chlorella algae, rather than plants and 8m2 of exposed Chlorella was enough to maintain a balance of CO2 and O2 for one person. The longest experiment in BIOS-3 was with a three-man crew for 180 days. The much more ambitious Biosphere 2 project in Arizona sealed in eight lucky people for two years in 1991. Biosphere 2 had planted-up land totalling to 8370m2 and the oxygen levels still declined steadily over the first 16 months to just 14.5 per cent.