America's Top Roadside Attractions
Wendy White / Alamy
This massive three-acre greenhouse was built as a research station to simulate the conditions of space colonization. Constructed to be airtight and self-sustaining, Biosphere 2 was the site of a two-year experiment with human “Biosphereans,” which went awry in 1991 when oxygen had to be pumped in, and subjects were sent crucial lifesaving supplies. Now managed by the University of Arizona and home to ongoing global-warming experiments, the self-contained mini-world with several climactic zones, including its own ocean with a coral reef and a rainforest, welcomes visitors.