What is Biosphere 2

I stumbled on something pretty cool today when I was watching a Youtube video, Biosphere 2. It is an artificial Earth system science research center that was built in the late 80s and early 90s. It was meant to be fully sustainable without any external resources. People were trying to build out a mini Earth ecosystem in this research center. During 1991 to 1993, a team of researchers and doctors were sent in the center for 2 years to build out a sustainable ecosystem. During that mission, the researchers had an incident which they almost ran out of O2. In the end, they had get external oxygen to continue the experiment. This is especially an interesting research as mankind will have to figure out a way to deploy a self sustain ecosystem when we try to colonize Mars someday.