The visible trench extends roughly twelve metres on an east-west axis, then bends northward for another four or five metres at its western end. What caused the collapse is the same thing that has emptied it: the stonework has been robbed out, carried off at some point by local hands who needed building material more urgently than they needed a buried passage.

A souterrain is an underground stone-lined tunnel or chamber, typically associated with early medieval ringforts and raths, the circular enclosed settlements that were the basic unit of rural life in Ireland from roughly the fifth to the twelfth century. They were used variously for storage, refuge, or ventilation of perishable goods, and their construction required considerable skill and labour. The rath at Cabraghkeel, recorded separately as SL010-009001, would once have enclosed a farmstead, and its souterrain, tucked into the south-western quadrant, would have been an integral part of that domestic world. The Ordnance Survey Name Books, which document local placenames and associated features from the nineteenth century, note the site, suggesting the depression was already a visible feature by the time surveyors passed through. By then, though, much of the dressed stone had long since been repurposed elsewhere, leaving only the shape of absence behind.