When the antiquarian T. J. Westropp recorded it in 1912, he called it a fort. What survives, however, is something rather more intimate: a roughly circular hut with an internal diameter of barely two and a half metres, and beneath it, a hidden underground passage.

The passage is a souterrain, a type of man-made underground tunnel built from drystone walling, common across early medieval Ireland and typically associated with settlement sites. They are thought to have served as places of refuge, cool storage, or concealment. At Cool, the souterrain opens from the north face of a revetted scarp and extends southwest for 4.5 metres, running directly beneath the hut floor. It is narrow, less than a metre wide and a metre high, with walls that corbel slightly inward towards a lintelled roof. What gives the site an unexpected precision is what fell through that floor over the centuries: limpet shells and other domestic debris dropped down into the passage below, and when radiocarbon dated they returned a result of 930 plus or minus 80 BP, placing occupation at roughly the tenth or eleventh century AD. Above ground, the hut itself is largely buried under sod, with only the tops of some slabs still visible and one large upright stone prominent in the southern wall. Immediately outside what may have been the entrance, on the north-north-east side, sits a small box-like structure measuring roughly 0.9 by 0.6 metres, its purpose unresolved.

The site sits in pasture on the northern cliff-edge of Valentia Island, a location that would have been exposed and wind-scoured even then. The combination of a tiny dwelling, an underground passage, a mysterious external box, and a dating derived from shellfish that fell through a floor makes this an unusually legible fragment of early medieval life, even in its ruined state.