A section of a lintelled roof collapsed inward, opening a hole in what had appeared to be ordinary farmyard pasture and revealing a curving underground passage that had gone unnoticed for centuries.

Souterrains are stone-lined underground passages or chambers associated with early medieval settlement in Ireland, probably used for cool storage or as places of refuge. The one at Aghamore is modest in scale but carefully made. The passage runs to 2.8 metres in length and just under a metre in width, with a maximum height of 1.19 metres, enough to crouch rather than stand. Its walls are built from very small, flat stones, none wider than 20 centimetres, laid horizontally and corbelled slightly inward at the top, a technique where courses of stone project gradually over one another to narrow the gap and bear the weight of the lintels above. The southeastern end of the passage terminates in a rounded wall, while the western end narrows into a low creepway, only 55 centimetres wide and 42 centimetres high, that has been blocked with yellow clay. Creepways of this kind were often used to connect chambers within a souterrain, or to provide a concealed exit point; this one has not been fully investigated beyond the clay blocking. The structure sits within a farmyard and its surroundings are flat agricultural land, which makes the sudden disappearance underground all the more striking.