Serpent's Hole, also called Poll na bPéist or the Wormhole, is a naturally formed rectangular sea pool cut into the limestone coast of Inishmore. Its shape looks almost engineered, which makes the Atlantic surge in and out of it feel even stranger.
The walk there crosses rough karst terrain, and the site is dangerous in bad weather or heavy swell. Go for the geology and the drama, not for casual swimming, and keep well back from the edges when the sea is moving hard.