Biosphere 2 is a 3.14-acre (1.27 ha) structure originally built to be an artificial closed ecological system in Oracle, Arizona (USA) by Space Biosphere Ventures, a company whose principal officers were John Polk Allen and Margret Augustine. Constructed between 1987 and 1991, it was used to explore the complex web of interactions within life systems. It also explored the possible use of closed biospheres in space colonization, and allowed the study and manipulation of a biosphere without harming Earth's. The name comes from the idea that it is modeled on the first biosphere, which is the life system on Earth. The funding for the project came primarily from Edward Bass' company, Decisions Investment, and cost $200 million from 1985 to 2007.

At a size comparable to two and a half football fields, it was the largest closed system ever created. The sealed nature of the structure allowed scientists to monitor the ever-changing chemistry of the air, water and soil contained within. The health of the human crew was continuously monitored by a medical team.

Inside was a 1900 square meter rainforest, an 850 square meter ocean with a coral reef, a 450 square meter mangrove wetlands, a 1300 square meter savannah grassland, a 1400 square meter fog desert, a 2500 square meter agricultural system, a human habitat with living quarters and office, and a below-ground level technical facility. Heating and cooling water circulated through independent piping systems, and electrical power was supplied from a natural gas energy center through airtight penetrations.

By 2006, the structure was no longer maintained in an airtight state, and the property, which is in exurban Tucson, was slated to be redeveloped for a planned community.

As of June 5, 2007, the property including surrounding land, totaling 1650 acres (668 hectares), was sold to a residential home developer for US$50 million. A development including homes and a resort hotel was planned for a portion of the land. The Biosphere itself will remain open for tours.

On June 26, 2007, the University of Arizona announced it would be taking over research at the Biosphere 2. The announcement ends immediate fears that the famous glass terrarium will be bulldozed. University officials said private gifts and grants will enable them to cover research and operating costs for three years with the possibility of extending that funding for 10 years.

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