There is nothing at ground level to suggest they are there: no earthwork, no hollow, no visible depression. The structure below is a souterrain, an early medieval underground passage or chamber system, typically cut from earth or built from stone, and associated with the ringforts and settled farmsteads of early Christian Ireland. At Reavouler, the version is earth-cut, with at least three separate chambers roofed by barrel-vaulted ceilings, and it sits in ordinary grazing land with no surface trace whatsoever.

The souterrain came to light in 1931, when it was investigated and reported on by someone named Somerville, whose account appeared in the Cork Examiner on 19th September of that year. The three chambers vary considerably in size. The largest, chamber 2, runs to nearly five metres in length and almost two metres wide, though its ceiling clears the floor by less than a metre, meaning anyone inside would have had to crouch or crawl. The other two chambers are smaller still. Connecting or separating the chambers are creepholes, low narrow gaps through which a person would have to squeeze, and at Reavouler several of these have been blocked: two at the northeast and southwest ends of chamber 1, and one at the southwest end of chamber 2. Why they were sealed, and when, is not recorded. Souterrains of this type were likely used for food storage, refuge, or both, their low temperatures and concealed entrances making them practical in a period when security and preservation were pressing concerns.