The accidental collapse opened a hole just large enough for a person to climb through, and what it revealed was a remarkably complete piece of early medieval engineering.

The structure is a souterrain, an underground passage or chamber built during the early medieval period, typically associated with ringforts and used for storage, refuge, or both. This one lies within a levelled rath, the remains of a ringfort whose earthen banks have long since been flattened into the surrounding landscape. The chamber itself is rectangular, running roughly four metres on its longer axis and only one metre across, with a height of about one and a half metres, just enough to crouch in. The walls are built from roughly coursed stone, and the roof is carried on seven large lintels, flat stones laid horizontally across the top of the walls. Five of those lintels remain in place; two have fallen inward. At the north-eastern end, a creepway leads further into the structure, a narrow passage barely half a metre wide and half a metre high, roofed with a single lintel, though it is now blocked by collapsed stones and cannot be followed. The creepway is a common feature of souterrains, designed so that anyone forcing entry would have to squeeze through blind and vulnerable, one at a time.

The chamber is accessible through the opening left by the original collapse, which measures roughly 1.3 metres by just over a metre. The blocked creepway means the extent of the souterrain beyond the main chamber remains unknown.