The Worm Hole

Poll na bPeist, the hole of the serpent, is a deep pool in the limestone shore platform below the cliffs of Inis Mor, and its striking feature is that it is almost perfectly rectangular - roughly the shape and proportions of a swimming pool, with clean vertical sides. It is natural. The limestone here is bedded in horizontal layers cut by two sets of vertical joints at right angles, and the sea has exploited that geometry, collapsing a section of a sea cave along the joint lines. It connects to the ocean through a submerged passage, so the water rises and falls with the swell. The Red Bull cliff diving series has used it. Getting there means a walk over broken limestone with no marked path and no signage, along cliffs with no fence, and it is easy to misjudge in poor visibility. In swell the pool washes over completely.

What you actually see

• The rectangle - a natural pool cut along the limestone's own joint lines.
• The connection - a submerged passage linking it to the open sea.
• The cliff platform - broken limestone shore below unfenced cliffs.
• The scale - larger than photographs suggest, and deep.

When to go

May to September, in calm conditions. In any swell the pool is dangerous and the platform washes. Ask locally for the route - it is unmarked.

Honest expectations

No path, no signs and no fence - people get lost or into difficulty here. In swell, waves break across the platform without warning. Swimming is done at your own risk with no rescue cover. Allow two to three hours from Kilmurvey.

Nearby

Dun Aonghasa is directly above; Kilronan and the ferry are east.