For 2 years, beginning in September 1991, four men and four women were locked inside this airtight 3-acre greenhouse in the desert, 35 miles north of Tucson near the town of Oracle. During their tenure in Biosphere 2 (Earth is considered Biosphere 1), they conducted experiments on how the Earth, basically a giant greenhouse, manages to support all the planet’s life forms. Today there are no longer any people living in Biosphere 2, and the former research facility is operated as equal parts science center and tourist attraction. Tours take visitors inside the giant greenhouse and into the mechanisms that helped keep this sealed environment going for 2 years. The futuristic building rising out of the desert hill country is an impressive sight.