A souterrain is an underground stone-built passage or chamber, typically associated with early medieval ringforts, and thought to have served for storage, refuge, or both. What makes this particular example quietly interesting is the detail preserved in its architecture despite considerable damage: a narrow entrance on the outer slope of the bank, barely half a metre wide and roofed by a single large lintel, opening into an oval chamber roughly the size of a large wardrobe, its ceiling formed by three heavy lintels. From there, a small creep, a deliberately tight connecting aperture less than half a metre in either dimension, leads further into the structure, requiring anyone passing through to crawl.

The souterrain was first discovered in 1818, and by 1867 it had attracted enough attention to warrant a formal examination, carried out at the instigation of a Markus Keane. The antiquarian Thomas Johnson Westropp described it in 1909, recording the overall length as 36 feet and noting that even then the roofing had already been robbed out and the chambers were badly overgrown. When inspectors visited in 2003, the situation had not much improved: brambles covered the area, portions of the passage were roofless with no stonework remaining, and recent clay fill was visible in places. Some digging to the northwest of the furthest chamber had clearly taken place at some point, though by whom and to what purpose is not recorded.