It is barely a third of a metre deep, just over ten metres long, and less than one and a half metres wide, with stones pushing up through the grass at intervals. To a passing eye it might look like subsidence, or the trace of an old drain. In fact, it is almost certainly the collapsed roof of a souterrain, an underground passage or chamber built during the early medieval period, typically used for storage, refuge, or concealment.

The depression begins at a gap in the southern bank of a rath, a circular earthen enclosure of the kind that served as a farmstead during the early medieval centuries in Ireland. Thousands of raths survive across the island, ranging from barely visible earthworks to well-preserved ringforts, and many contain souterrains associated with the families who lived within them. At Doonflin, the line of the feature enters through that gap in the enclosing bank and curves slightly towards the north-north-west as it moves into the interior of the enclosure. The stones visible along its length are likely the remnants of the dry-stone lintels or side walls that once formed the roof and walls of the passage, now pushed up or displaced as the ground above settled into the void left by the collapse.