I remember when this huge experimental complex was being built in the late 1980s. Privately funded by a wealthy Texan, Biosphere 2 was to be a "closed system," a place that would be sealed off from the rest of the world. Everything needed for human habitation would be produced inside. In the early 90s, eight "Biospherians" entered the complex and the doors were sealed. Inside, the four men and four women would grow their own food, plants would produce oxygen, and everything would be recycled just like on the earth itself, which the Biospherians called "Biosphere 1."
One of the hopes was to learn how human astronauts might exist on lengthy, self-supported flights to other planets. I seem to recall that some things went wrong almost from the start, and that oxygen from the outside had to be pumped in to keep the Biospherians going. When I visited the Biosphere in the Arizona desert near Oracle, I wasn't prepared to be so impressed. But I was.
First off, it was explained to our tour group that what the media called a failure at the time was far from it. The only assistance from the "outside," was an injection of oxygen on two brief occasions when the Arizona sun inexplicably didn't provide enough shine for the plants inside to produce enough.
Aside from that, the Biosphere performed as had been planned, and is still providing a unique platform for scientific study. Ownership of the private complex has changed a few times, and it is now managed by the University of Arizona. Experiments in human habitation, botany, climate, and other sciences take place there all the time.Biosphere 2 contains five distinct "biomes" -- a rainforest, an ocean with a coral reef, a desert, a grassland, and a mangrove wetland. It's hard not to be amazed by a huge glass-enclosed rainforest with fully grown trees and plants.
It's also curious to come in from the dry Arizona desert outside the complex and immediately feel the humidity of the rainforest. Everything in each of the biomes, including temperature, humidity, chemical makeup of the air, and "rain" falling from pipes way up on the ceiling, is carefully controlled. We were taken to a level beneath the ground and shown gargantuan machines that oversee all aspects of the indoor environment. The Biospherians had to maintain this equipment as well as do farming and other chores above ground. 
The Biospherians indeed did finish out their two-year assignment and another group went in for a later stint. These days Biosphere 2 is no longer a closed system, but rather is used for a multitude of independent experiments by the University of Arizona science departments as well as visiting students and scholars. It is an impressively cool place.
