What were the problems with Biosphere2?

Answer 1

When Biosphere2 was constructed near Tucson, Arizona, many tons of concrete were poured in the construction process. For many months after it sets, concrete absorbs carbon dioxide as part of the curing process. Plants, animals and humans were sealed in the environment before the concrete was completely cured. The organisms, including the humans, were releasing carbon dioxide during respiration which, normally, would have been absorbed by the plants that would then release oxygen into the environment, However, most of the carbon dioxide was being absorbed by the concrete so the plants could not flourish and provide oxygen to the inhabitants. The oxygen levels dropped to a dangerous level and nearly killed the humans living there. Supplemental oxygen had to be provided by the scientists to prevent a disaster.