The interior of the south "lung". There are two of these structures that regulate air volume inside the Biosphere. When air is removed from B2 it is pumped here and a bladder membrane elevates and when air is needed again it is pumped out and the bladder collapses, In the centre is a deep resevoir to store excess water until it is needed. There is a round plate with legs that descends and covers the resevoir when the bladder is deflated.