Biosphere 2 needed something like a bellows, a lung!
Biosphere 2 would heat up like a greenhouse and cool during the night and during cold, cloudy days. The expansion and contraction would subject the rigid steel and glass structure to enormous pressures as the air inside expanded and contracted. On a hot day, pressure would push out. On a really cold day, Biosphere 2 might implode.

Bill Dempster, Director of Engineering Systems, had an inspired solution, and in time Biosphere 2 got a pair of lungs, or "variable expansion chambers". The two lungs took the form of graceful hemispheric white domes which protected the liner from wear and tear of the Sun. Both domes, 150 feet away from Biosphere 2 had underground air tunnels connecting them to the main structure. Inside each tank and connected to each tunnel, a gargantuan synthetic rubber membrane with a circular metal top moved freely up and down on a cushion of air.
As air inside Biosphere 2 expanded from the Sun's heat, it flowed through the tunnels and into the lung, raising its top. As air cooled, the lungs deflated.