In 1991, four men and four women sealed themselves into a $150 million science project called Biosphere 2. (Biosphere 1 is the Earth.) Their goal was to spend two years in an airtight, self-contained environment, growing their own food and recycling their air and water. The experiment didn't go exactly as planned--an unusually cloudy year resulted in reduced photosynthesis, and oxygen from outside needed to be pumped in. Still, the Biosphere 2 is considered a modern engineering wonder.