Fumaroles are vents from which volcanic gas escapes into the atmosphere. Fumaroles may occur along tiny cracks or long fissures, in chaotic clusters or fields, and on the surfaces of lava flows and thick deposits of pyroclastic flows. They may persist for decades or centuries if they are above a persistent heat source (active Magma chamber) or disappear within weeks to months if they occur atop a fresh volcanic deposit that quickly cools. The teperatures rise from 70 C - 100 C or more. In some cases they are hidden in the ground (like in Stephanos-Crater on Nisyros) and you can break into them. The gases are dangerous and a gas-mask is often needed. They are alwas a sign of active volcanism.