A souterrain is an underground stone-lined passage or chamber, typically associated with early medieval ringforts, where it served as a place of refuge, storage, or concealment. At Shanaknock, the above-ground fort survives, but the passage beneath it has been blocked up for the better part of a century, and the interior of the enclosure is now so overgrown that there is nothing left to see at the surface.

The story of what was lost here comes in two stages. Writing in 1934, a researcher named Bowman recorded simply that a souterrain had existed within the fort but was by then already filled in. A few years later, in 1937, a writer named Broker added a detail that shifts the picture slightly: flagstones found in the water table of the adjacent railway may indicate that the passage once extended further south, into ground that the railway construction had by then disturbed or destroyed. The southern edge of the ringfort itself was destroyed by the railway, so what Broker observed may have been structural evidence of the souterrain's continuation into that lost section. It is a small, inconclusive clue, but the kind that tends to linger.