The one at Castleventry in County Cork came to wider attention not through excavation or fieldwork but through accident: in 1991, the ground in a farmyard simply gave way, opening a hole roughly 1.6 metres by 1.2 metres, and dropping anyone curious enough to look down into a previously unknown chamber that had sat sealed beneath the soil for centuries.

What the collapse revealed was a subrectangular earth-cut chamber, about 3.7 metres long and 1.4 metres wide, with a rounded roof and a floor cut into the underlying bedrock. The southern end narrows and turns slightly to the south-west, where a blocked-up construction shaft, less than a metre wide and under a metre high, marks one of the practical necessities of building underground: a shaft sunk to allow the spoil to be removed during digging, then sealed once the work was done. A small opening in the western wall, only 0.5 metres wide and 0.4 metres high, may have served as a creepway connecting to a further passage, or as an air-vent; collapse and infilling had made it impossible to determine which. The souterrain sits roughly 100 metres north of a rath, the circular earthwork enclosure that would have been the associated farmstead or settlement above ground, and the proximity makes clear these two features were once part of the same complex. Nor was the 1991 collapse the first. Local memory recalled an earlier subsidence, backfilled immediately, about 2.5 metres to the south-west. That earlier opening had briefly exposed a larger, subcircular chamber with two tunnels extending from it, one to the south-west and one to the south-east, along with what appeared to be a further construction shaft and two air-vents before the ground was closed up again.