A souterrain, the term for an underground chamber or passage typically built during the early medieval period and associated with nearby settlements or fortified enclosures, once sat at the heart of this site. When it was first recorded in the mid-nineteenth century, it had already yielded something unsettling: enormous quantities of bone, teeth, and charcoal, the kind of deposit that raises more questions than it answers.

The earliest description comes from Caulfield, writing between 1847 and 1850. He recorded a cavity roughly six feet in diameter, with two circular ends joined by a rectangular passage running fourteen feet and nine inches in length. By the time McCarthy investigated in 1976, the chamber had become inaccessible. What he found instead were four sealed entrances distributed across the ringfort, at the centre, and at the western, southern, and northern edges. The northern entrance was particularly notable: a low, earth-cut creepway, the kind of narrow passage a person would have to crawl through, set within an oval embanked area. That deliberate, low-slung design is characteristic of souterrains generally, where restricted access was likely part of the point, whether for defence, storage, or concealment. Today there is no visible surface trace of any of it.