Drawings of human colonies on other planets often picture the entire community under a glass or plastic bubble. The bubble is intended to create an atmosphere with adequate oxygen and other essential elements. But similar bubble-like structures have also been constructed on earth. One of the most famous, and controversial, is a site in the Arizona desert. Biosphere 2, as it is called, was built not far from Tucson in 1984 and is now run by Columbia University. This huge(7,200,000-cubic-foot)glass and steel construction contains several separate ecosystems, including a desert, a rain forest, and a 900,000-gallon "ocean." The climatic conditions-humidity, temperature, air quality-are regulated by sensors and can be adjusted as needed or desired. For example, a rainstorm can be created to increase the humidity. The adjustable features of Biosphere 2 make it an ideal location to perform experiments to help determine the effects of such climatic changes as global warming. The current conditions at Biosphere 2 are vastly different from those in 1993, when eight people who had moved into the environment with great fanfare two years earlier moved out in failure. Though promising to be self-sufficient , these "colonists" had so much trouble regulating the environment that they reportedly had food smuggled into them. Oxygen levels became dangerously low; most plants and animals died. In taking over the unsuccessful site, Columbia hopes to erase its notorious past by focusing on small research projects that gradually answer some of Biosphere 1's -- that is, Earth's most basic environment questions. This passage primarily deals with   _  .
conditions of life in Biosphere 2 building controlled environments on other planets why Biosphere 2 failed in the past what makes a good biosphere colonist