Systems and methods for extracting hydrocarbon gas utilize a vacuum chamber (38, 64) with a mud chamber portion (92) that is expandable and contractible. Gas is extracted at vacuum pressures. A method of extracting hydrocarbon gas from hydrocarbon-bearing drilling mud comprises flowing hydrocarbon-bearing drilling mud into a degassing chamber (38, 64) of a degassing device (10, 60) and expanding a mud chamber portion (56, 92) of the degassing chamber to extract hydrocarbon gas from the hydrocarbon-bearing drilling mud.