The depression, measuring up to twenty-two metres in length, about 1.8 metres wide at its base, and sinking roughly 1.2 metres into the earth, follows a line from the inner edge of the southern bank to a gap on the west-southwest side. The working interpretation is that this is a collapsed souterrain, an underground passage or chamber built, typically in early medieval Ireland, for storage, refuge, or both. Constructed from stone-lined tunnels and roofed with large slabs, souterrains were common features of ringfort settlements, and when the roof eventually gives way the result is exactly this kind of elongated hollow in the surface of the ground.

The ringfort itself sits on a natural rise in gently undulating landscape, with clear views stretching out in all directions, the kind of position that would have made obvious sense to anyone choosing a defensible or simply practical place to settle. Ringforts, which are roughly circular enclosures defined by earthen banks and ditches, were the most common form of rural settlement in early medieval Ireland, and the presence of a possible souterrain within the southwestern quadrant of this one fits a well-established pattern. The association of underground passages with these enclosures is well documented across the country, though at Petitswood the feature remains tentative, described as something that may represent a collapsed example rather than a confirmed one. That uncertainty is itself worth noting: the archaeological record is full of such careful qualifications, where the surface evidence points strongly in one direction without quite closing the case.