mofette

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An opening in the earth from which carbon dioxide and other gases escape, usually marking the last stage of volcanic activity.
2. n. The gases escaping from such an opening.

Century Dictionary and Cyclopedia

1. n. An irrespirable gas escaping from the earth; a gas-spring. It is sometimes (although rarely) applied by writers in English to carbonic-acid gas escaping from the rocks in regions of nearly extinct volcanism, and, by extension, to the openings from which this gas escapes. The mofettes are analogous to the soffioni or “blow-holes,” but betoken a still further advance of the region toward complete extinction of the volcanic forces.