A pressure compensating lung balances pressure between open and closed environments which are substantially isolated from and independent of each other, for example, the Earth's open environment and the closed environment of a second, smaller closed ecological system, Biosphere 2. The lung has an airtight housing and a barrier which includes a flexible membrane, one side of which faces Biosphere 2 and the other side of which faces the outside, with a central pan the perimeter of which is surrounded by the membrane. The membrane has approximately the shape of a surface of a torus. It deviates from a toroidal shape sufficiently to avoid substantial circumferential tension in the membrane. The weight of the pan exerts a positive pressure on the atmosphere in Biosphere 2 for preventing inward leaks. The pan and membrane are protected by a weather cover which can be sealed against the outside environment so that a fan can affect the air pressure from the outside acting on the membrane, and hence the pressure difference between the inside of Biosphere 2 and the Earth's atmosphere.