Blowhole

Noun

1. The spiracle, on the top of the head, through which cetaceans breathe.
2. A vent for the escape of gas.
3. A top-facing opening to a cavity in the ground very near an ocean's shore, that leads to a marine cave from which wave water and/or bursts of air are expelled.
4. An unintended cavity filled with air in a casting product.
5. A vertical opening in the top of computer cases, that let hot air, primarily from the CPU heat sink, escape quickly.