A balloon having a balloon envelope that is fillable with a first lifting gas, a top plate arranged at a top of the balloon envelope, an outlet port that provides a passageway from an inside of the balloon envelope to the atmosphere, an inner bladder positioned within the balloon envelope and positioned at the top of the balloon envelope and coupled to the top plate, wherein the inner bladder is fillable with a second lifting gas, wherein the outlet port is configured to vent the first lifting gas to the atmosphere while keeping the second lifting gas in the inner bladder.