A souterrain is an underground stone-lined passage or chamber, typically built during the early medieval period and associated with nearby settlement enclosures such as ringforts; they were used variously for storage, refuge, or both. At Gneeves, the souterrain's existence is inferred rather than confirmed, recorded as lying within the bounds of a ringfort but leaving no visible trace on the surface above it.

The only physical evidence that anyone has looked is a low mound, roughly three metres by two, sitting to the south-east of the ringfort's centre. That modest rise in the ground is not a natural feature; it is the spoil left behind by previous digging, the accumulated earth from an attempt to locate the passage below. Whoever dug there did not find what they were looking for, or at least did not find it conclusively, and the site has been recorded ever since as a souterrain with no visible surface trace. It occupies a strange category in the archaeological record: present enough to be catalogued, absent enough to remain unverified.