In 2003, the ground at Coolcraheen gave way under the weight of the equipment, opening a hole roughly two metres across and one and a half metres deep, and revealing beneath it an ancient underground chamber that had been quietly waiting, sealed by its own collapsed ceiling, somewhere beneath the working fields of County Cork.

What the collapse exposed was a souterrain, the term used for the stone- and earth-cut underground passages and chambers built throughout early medieval Ireland, most commonly in association with ringforts. They served variously as storage spaces, refuges, or places of concealment. The Coolcraheen example sits within what may be a rath, a type of ringfort defined by earthen banks, though the enclosure itself remains uncertain. The chamber uncovered in 2003 is oval and cut directly into the earth, with walls that curve inward toward a roof which had partially collapsed before discovery. Fragments of flat stone near the apex suggest that lintels, large horizontal slabs laid across the top of the structure, once formed part of the ceiling. Two passages lead off from the central chamber: one running east, narrowing as it goes, its floor buried under debris and its walls punctuated by small recesses on each side; the other heading northwest, sloping downward to a depth of nearly one and a half metres below ground level and largely blocked by fallen material. At the eastern end of the eastern passage, a construction shaft is still visible, a feature sometimes left behind from the original building process, where workers would have dug down from the surface to excavate sections of the tunnel from above before covering it over again.

The site was investigated by Ryall in 2003 following the accidental discovery, though the extent of the damage caused by the initial collapse, and the degree to which the structure remained intact beyond what was then visible, was not fully resolved. Both passages were obscured by debris, and much of what lies furthest from the central chamber remains unexamined.