Yet at Carrowgun in County Sligo, this depression, roughly two metres wide and thirteen metres long, is understood locally to mark the buried course of a souterrain, an underground passage or chamber of the kind commonly associated with Early Medieval settlement in Ireland. Souterrains were typically built from stone or timber, roofed over, and used for storage or refuge, and over centuries the ground above them can slowly subside, leaving exactly the kind of linear hollow visible here.

The depression sits within a rath, the circular earthen enclosure that was the standard form of rural farmstead across Ireland from roughly the sixth to the twelfth century. A rath typically consists of one or more banks and ditches enclosing a domestic space, and souterrains are a fairly common feature within them, though the majority have long since collapsed, been quarried for stone, or simply faded from visible record. What makes the Carrowgun example worth noting is that its presence has been carried forward not through excavation or formal documentation but through local tradition, the kind of accumulated community knowledge that frequently preserves details that official survey misses or arrives at only later.