Biosphere 2 was built just north of Tuscon, AZ, in the early 1990's as an enclosed environment to test the possibility of people living in a sealed, self sustaining, living environment. Eight people actually lived inside Biosphere 2 for two years however the atmosphere had to be supplemented. It was determined afterward that the curing of the massive amounts of concrete used to build Biosphere was absorbing the oxygen from the air. Check it out if you are ever in this area, it's worth the price.