Vent is a general term for an opening in the surface of the Earth’s crust, such as a mofette or solfatara (each a kind of fumarole), through which molten material or gas is forced during volcanic activity. Vents may consist of a single circular-shaped structure, a large elongate ﬁssure and fracture, or a tiny ground crack. A vent can also be the conduit or channel through which material is extruded. The term also applies to an opening that releases gas during the burning of underground coal seams. Burning Hills in Grand Staircase–Escalante National Monument offers stunning examples of dark red and black surfaces pitted with vents from the burning of underground coal seams ignited by lightning strikes.