The giant Plexiglass bubble known as BIOSPHERE 2 looms out of the desert thirty miles north of Tucson, in a high-security compound at mile marker 96.5 on Hwy-77 and near the town of Oracle . On its completion in 1991, this vast complex was trumpeted as a major laboratory for experiments in applied environmental science. Containing five separate "biomes," or self-contained ecosystems - rainforest, marsh, savannah, desert and a 25ft-deep ocean - it was designed as a miniature working model of Biosphere 1, planet Earth itself, and stocked like a real-life Noah's Ark with almost four thousand species of plants and animals. Backed by a hundred million dollars' worth of support from Texas oil tycoon Ed Bass, this far-fetched scheme was intended as a step toward the eventual colonization of Mars.

Eight "Biospherians" were sealed into Biosphere 2 in September 1991, their "mission" being to survive in isolation for two full years. This they more or less did, although emergency oxygen twice had to be pumped in as carbon dioxide levels rose. Much of what transpired was replete with irony; hungry Biospherians soon found themselves planting bananas and papayas in what was supposed to be the inviolate wilderness of the rainforest, and destroying parts of the desert to boost oxygen, while the ocean proved impossible to keep clean. By the time the next crew moved in, conditions at Biosphere 2 had degenerated into farce. Two of the original crew, who had been acrimoniously fired, sneaked back and broke the Biosphere's seals from the outside, thus aborting the second mission after just six months; they later sued for wrongful dismissal and won compensation. Since 1996 Biosphere 2 has been taken over by Columbia University, which seems to be frantically trying to think of something to do with its vast new "campus."

In its brief heyday, Biosphere 2 was one of Arizona's most popular tourist destinations. Now that no one is locked inside, the crowds have dwindled, but the two-and-a-half-hour guided tours are still worth taking (daily 8am-5pm; $12.95; ). Most of that time is spent peering into the greenhouses hoping to spot any sign of life larger than an ant, and marveling at the fact that, whatever you may have assumed, Biosphere 2 is not solar powered but depends on an external natural-gas power plant. Visitors are also allowed into the Biospherians' futuristic living quarters, tacked like a space capsule onto the back of the main block, where they appear to have suffered no privations whatsoever. For an extra $10, a 45-minute "Under the Glass" tour gets you through the massive airlock doors and into the giant terrarium itself.

The all-day Cañada del Oro restaurant serves good meals on an appealing terrace. There's also a cybercafé, and good-value lodging at the on-site Inn at Biosphere 2 (tel 520/825-6222; May-Sept $50-75, Oct-April $100-130) - especially worth it if you come for one of the new on-site astronomy programs (an observatory was added in 2001) and don't want a late-night drive back into town.

Biosphere 2

• Biosphere 2

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