It is grassy, irregular, and easy to walk past without a second thought. But the shape it traces follows the outline of a souterrain, an underground stone-lined passage of the kind built in early medieval Ireland, typically for storage, refuge, or concealment. Most souterrains survive as earthworks exactly like this one, their roofs long since collapsed, their interiors silted up. Here, at the north-eastern end of the shorter arm, a single fallen roof lintel is still visible, the last legible fragment of what was once a roofed underground structure at least 4.25 metres long, and possibly considerably more.

When the scholar and antiquarian M. Costello examined the site in 1903, he noted it was so filled as to be impossible to examine internally. Even then, the passage had been reduced to little more than a rumour in the earth. What he did record, and what makes this particular souterrain worth pausing over, is the local belief he encountered: that a communication existed between this underground passage and Castlegrove Castle, which lies approximately 450 metres to the south-east. Whether that tradition reflects a genuine physical connection, a folk memory of some now-vanished feature, or simply the kind of story that attaches itself to old stones and dark underground spaces, cannot now be determined. By 1932, the Ordnance Survey's third edition of the six-inch map was naming the spot simply as 'Souterrain', which suggests the feature had enough local presence to be worth labelling, even as its interior became inaccessible. The hollow as it exists today measures 13.5 metres in total length, running north-east to south-west along its shorter arm before turning sharply westward.