Biosphere 2 is an artificial, materially closed ecological system. Why does it have the number 2 in its name?

Biosphere 2 is an American Earth system science research facility located in Oracle, Arizona. It was originally constructed between 1987 and 1991 and has been owned by the University of Arizona since 2011. Its mission is to serve as a center for research, outreach, teaching, and lifelong learning about Earth, its living systems, and its place in the universe. It remains the largest closed system ever created.

Biosphere 2 was originally meant to demonstrate the viability of closed ecological systems to support and maintain human life in outer space as a substitute for Earth's biosphere. It was designed to explore the web of interactions within life systems in a structure with different areas based on various biological biomes.

In addition to the several biomes and living quarters for people, there was an agricultural area and workspace to study the interactions between humans, farming, technology and the rest of nature as a new kind of laboratory for the study of the global ecology. Its mission was a two-year closure experiment with a crew of eight humans ("biospherians"). Long-term it was seen as a precursor to gain knowledge about the use of closed biospheres in space colonization.

As an experimental ecological facility, it allowed the study and manipulation of a mini biospheric system without harming Earth's biosphere.