In 1991, eight people sealed themselves inside an artificial, self-contained ecosystem in Arizona as part of an ambitious experiment to live without outside support. Over the next 16 months, oxygen levels fell from 21% to just 14% after carbon dioxide unexpectedly reacted with and became trapped in the facility’s concrete, disrupting the carefully balanced atmosphere. The experiment was known as Biosphere 2, a huge glass-and-steel facility built near Oracle, Arizona, to test whether people could survive in a completely sealed ecosystem. Designed as a miniature version of Earth, it contained a rainforest, ocean, desert, wetlands, farmland, and living quarters. On September 26, 1991, eight crew members entered the enclosed habitat for a planned two-year mission. The project was promoted as a step toward future space colonies and self-sustaining living, but it soon faced major challenges. Crops produced less food than expected, the crew lost weight, interpersonal tensions grew, and oxygen levels steadily declined. Researchers later discovered that soil microbes were consuming oxygen while producing carbon dioxide, much of which reacted with the concrete walls and became trapped as calcium carbonate. As a result, oxygen disappeared without a corresponding buildup of carbon dioxide, forcing the team to pump oxygen into the habitat to complete the mission. Today, Biosphere 2 remains in operation and is owned by the University of Arizona, where it serves as a research center for climate science, Earth systems, and environmental studies.(Historic Vids)