Fumaroles (from the Latin word _fumus_, meaning “smoke”) are those
features that discharge only steam and other gases such as carbon
dioxide and hydrogen sulfide; hence, they are commonly called “steam
vents.” Usually these features are perched on a hillside or other high
ground above the level of the flowing springs. In many fumaroles,
however, water can be heard boiling violently at some lower, unseen
level.