Many lakes whose existence depends wholly on rainwater runoffs are seasonal. The phenomenon is not particularly mysterious—the lake forms when rainwater accumulates in a depression, and disappears when the water dries off. However, in Ireland, there is a type of disappearing lake that does not evaporate into the air; it drains underground. They are known as turloughs.

Lough Bunny in The Burren, County Clare, Ireland, is a turlough. Every summer it drains into fissures around the lake's northern end. Photo: Tom Fahy/Flickr