A fumarole ( Latin fumus , smoke ) is an opening in the crust of the Earth , often in the neighborhood of volcanoes , where steam and gases come out , for instance carbon dioxide , sulfur dioxide , hydrochloric acid , and hydrogen sulfide . A fumarole ( Latin fumus , smoke ) is an opening in a planet 's crust , often in the neighborhood of volcanoes , which emits steam and gases such as carbon dioxide , sulfur dioxide , hydrochloric acid , and hydrogen sulfide .