Poll na bPéist — the Worm Hole — is one of the strangest natural features in Ireland: a perfectly rectangular pool in the limestone cliffs on the southern coast of Inis Mór, carved by the Atlantic into a shape so geometrically precise it looks man-made. The pool is roughly 30 by 10 metres, with vertical limestone walls, and connects to the open sea through a submerged tunnel. In calm conditions the water is a vivid turquoise; in rough weather the waves surge in and spray the surrounding ledges.
Reaching it requires a thirty-minute walk west from Dún Aonghasa on rough limestone paths along the cliff top, or a shorter walk from the road near Bun Gabhla. The approach is unmarked and the cliff edge requires care — this is not a place for inattention. But arriving at the pool, with the Atlantic hammering the coast around it and this extraordinary rectangle of contained turquoise water in the rock, is one of the island's great moments.