The entrance was covered over by 1939, and the ground above shows only a vague unevenness, the kind that might be mistaken for ordinary agricultural disturbance. There is nothing to see at the surface, which is part of what makes this place quietly compelling.

A souterrain is an early medieval underground structure, typically stone-built or rock-cut, believed to have served as a place of refuge, storage, or both. The one at Nunstown was recorded in some detail by Hitchcock in 1854, when it was apparently still accessible. The description he left is precise enough to reconstruct the experience of entering it: a low east-to-west passage roughly five metres long, built partly from stone and partly cut directly into the bedrock, with flat lintels overhead and barely enough width to pass through comfortably. At the far end, a creepway, a deliberately narrow connecting gap designed to slow or obstruct an intruder, measured just sixty centimetres high by forty-five centimetres wide, and led into a circular corbelled chamber. Corbelling is a technique in which courses of stone are laid so that each projects slightly inward over the one below, eventually closing to form a domed roof without the need for a keystone or mortar. The chamber itself was modest but enclosed, roughly one and a half metres wide and not quite two metres long, with walls rising to about the same height. Among the things found inside were small bones and bone fragments, though no further detail about their origin or significance was recorded at the time.