This souterrain, marked simply as ‘Cave’ on an 1837 Ordnance Survey map, stretches for about 14 metres in a gentle arc, weaving its way through the earth from southwest to north to southeast. The structure vanished from later maps, suggesting it fell out of local memory or use, though its preservation order status since 1973 has ensured its protection for future generations.

The souterrain consists of three interconnected chambers, each showcasing the impressive drystone construction techniques of early medieval Ireland. Builders carved parts directly into the bedrock whilst constructing walls without mortar and capping them with massive stone slabs. Entry is now possible through a collapsed roof section that leads into the southeastern chamber, a subrectangular space measuring 2.7 metres long and 2.1 metres wide. From here, visitors must crawl through a narrow creepway to reach the central passage; significantly longer at 5.5 metres but only 75 centimetres wide; before another creepway opens into the final oblong chamber. This western chamber features an intriguing floor design of large stones overlaid with smaller ones, possibly for drainage or comfort.

Perhaps the most mysterious features are the circular depressions found on the roof slabs; over twenty mark a large slab in the southeastern chamber, with three fainter examples in the middle section. These cupmarks, whether natural formations or deliberate carvings, add another layer of intrigue to this subterranean refuge. Such souterrains served multiple purposes in early medieval Ireland, from storage spaces for food and valuables to hiding places during raids, and this example at Creggan stands as a well-preserved testament to the engineering skills and daily concerns of those who built it centuries ago.