India, Sept. 29 -- Eight people entered the facility called Biosphere 2 on September 26, 1991, after sealing up the 3.15-acre glass-enclosed facility in southern Arizona from the outside world. The facility had been built to study entire ecosystems and investigate how life support could be provided in space. But then the air changed inside Biosphere 2, and solving the mystery required a closer investigation.

The clearest account came out in a 1994 paper titled Oxygen Loss in Biosphere 2, published in EOS, Transactions American Geophysical Union by Jeffrey P. Severinghaus, Wallace S. Broecker, William F. Dempster, Taber MacCallum and Martin Wahlen. The study reported that oxygen in Biosphere 2 had fallen from the ambient 21% to 14% during...