What begins as a souterrain, an underground stone-lined tunnel of the kind built in early medieval Ireland for storage or refuge, transitions after roughly five metres into something far older: a natural cave. A low drystone wall marks the boundary between the two, built at the point where human construction ends and geology takes over, as though whoever made it felt the need to acknowledge the threshold.

The site was discovered in 2003 and is sometimes referred to as Poll Rannagh East Cave, though it sits in the townland of Termon. The souterrain section is built in the characteristic manner, with drystone walling on either side and large flat slabs laid across the top to form a roof. Beyond the dividing wall, the cave passage descends and is coated with moon milk, a soft white mineral deposit formed by calcite crystals, which gives the walls a pale, almost powdery appearance. The passage extends to a total length of twelve metres before it becomes choked with boulders. When the site was first explored, a number of human bones were found at the back of the cave. On a follow-up inspection in 2005, those bones were no longer visible, which raises questions that remain, for now, unanswered.

What makes this place quietly compelling is the relationship between its two parts. Whoever constructed the souterrain did so against the mouth of a pre-existing natural cavity, effectively borrowing its entrance and extending the usable underground space. Whether the cave was already known to have bones in it at the time of construction is impossible to say. The combination of deliberate architecture and natural formation, separated by a single low wall, gives the site an unusual character that goes beyond either element on its own.