Blowhole

1. A cavern in a cliff, at the water level, opening to the air at its farther extremity, so that the waters rush in with each surge and rise in a lofty jet from the extremity.
2. A nostril or spiracle in the top of the head of a whale or other cetacean.
3. A hole in the ice to which whales, seals, etc., come to breathe.
4. (Founding) An air hole in a casting.

There are two spiracles or blowholes in the common whales, but only one in sperm whales, porpoises, etc.