A hole for the escape of gas or air WordNet 3.0 & Open English WordNet

In geology, a blowhole or marine geyser is formed as sea caves grow landward and upward into vertical shafts and expose themselves toward the surface, which can result in hydraulic compression of seawater that is released through a port from the top of the blowhole. Wikipedia

A hole at the inland end of a sea cave Wikipedia Disambiguation

A vent for the escape of gas. Wiktionary

Vent for the escape of gas. Wiktionary (translation)

A hole through which gas or air can escape. Wiktionary