Biosphere 2

One of the oddest experiments of the 20th century began 25 years ago tomorrow. Eight volunteers sealed themselves inside Biosphere 2, a miniature replica of Earth built in the desert north of Tucson. It was designed to study environmental problems confronting Earth itself. But a secondary goal was to look at how we might build self-sustaining colonies on other worlds.
The structure itself looked like a cross between a Space Age greenhouse and an ancient Egyptian monument. A multi-level steel framework was covered with thousands of panes of glass. Inside, it included an artificial mountain, an ocean, and livestock pens. About 3800 species of plants and animals were scattered through a desert, a rainforest, and other habitats.
In the first major experiment, the eight “biospherians” were to live in the structure for two years, completely sealed off from the rest of the world.
Yet things didn’t work as planned. Carbon dioxide built up in the air, so they had to use chemicals to remove it.