Inishmore, the largest of Ireland’s Aran Islands, is home to a remarkable natural wonder, a rectangular pool cut so straight into limestone that it looks man-made.

Also known as “The Wormhole” or “The Serpent’s Lair”, Poll na bPéist is a natural water basin with an edge length of approx. 10 by 25 meters within a stone formation. It can only be accessed by walking along the cliffs south of the ancient site Dún Aonghasa, but in recent years it has become famous for hosting the renowned Red Bull Cliff Diving World Series. The most fascinating thing about Poll na bPéist is its remarkable rectangular shape, which has led many to question its natural origins and sparked several theories, including that it is the work of an ancient civilization.

Photo: Marc Calhoun / The Worm Hole / CC BY-SA 2.0

The unusual formation features underwater channels that connect to the sea. Water flows into Poll na bPéist through a subterranean cavern, or sometimes waves crash over at high tide, filling it from the top. The pool is around 300 meters deep and features underground channels through which the currents surge and retreat from the open sea, threatening to suck unsuspecting swimmers into the ocean.

As hard as it seems to believe, Poll na bPéist formed naturally, through the erosion of deeper layers of limestone along straight edges.

For more amazing natural rock formation, check out the Al Naslaa, in Saudi Arabia’s Tayma Oasis.