Poll na bPéist, natural swimming pool in Ireland

Poll na bPéist is a rectangular natural pool about 25 metres long and 10 metres wide, cut into the limestone coast of Inis Mór in the Aran Islands. Its straight edges follow right-angled fractures in the rock, and an underground channel links it to the Atlantic, so its floor is covered by seawater.

Poll na bPéist is a karst pit formed by erosion of deeper limestone layers along straight fracture lines, giving it a rectangular shape of about 25 by 10 metres. Its floor is flooded by the Atlantic through submarine tunnels. Since 2014 it has hosted a Red Bull Cliff Diving World Series event.

The Irish name Poll na bPéist means "worm's hole" or "serpent's lair"; English speakers call it The Wormhole or The Serpent's Lair. Since 2014 the pool has been a stop on the Red Bull Cliff Diving World Series, when divers compete by leaping into the rectangular opening from the surrounding limestone.

The site sits on the west side of Inis Mór, about 1.6 kilometers from Dún Aonghasa, and is reachable by unpaved uneven paths marked with red painted rocks. Wear sturdy shoes and allow time for a careful walk, as the cliffs are unfenced and rocks can be loose and slippery.

The pool hosted a stage of the Red Bull Cliff Diving World Series in 2014, not in 2012 or 2017. Its regular right-angled outline comes from fracturing and karst processes in the limestone, not from human cutting, and the pool is about 25 metres long and 10 metres wide, not deep.