fumarole

A vent in the earth's surface from which volcanic gases and steam escape — a hole in the ground that breathes hot, sulfurous air. Fumaroles are found on active volcanoes, in geothermal fields, and near hot springs, marking the places where the planet's internal heat reaches the surface. The gases — mostly water vapor, carbon dioxide, and hydrogen sulfide — give fumaroles their distinctive rotten-egg smell. They hiss, whistle, and roar, depending on pressure.