A souterrain, an underground stone-lined tunnel or chamber typically built during the early medieval period, often for storage or refuge, lay completely undetected beneath the soil until 1985, when the landowner began digging foundations for a garden wall and broke through into something far older.

The structure is built in drystone construction, meaning the walls are fitted together without mortar, and roofed with flat lintel stones laid across the top. That it survived undetected is striking on its own, but the absence from every edition of the Ordnance Survey six-inch maps adds a further layer of obscurity. These maps, produced from the 1830s onward and revised at intervals, recorded an enormous amount of archaeological and topographical detail across Ireland, and a souterrain appearing on none of them suggests it was either forgotten very early or never widely known at all. After the 1985 discovery, rather than leaving the structure open, it was sealed again and is now inaccessible. What makes the location still more interesting is that a second possible souterrain lies approximately fifty metres to the south, situated within a rath, which is a circular earthen enclosure used as a farmstead in early medieval Ireland. The proximity of the two suggests this corner of Corimla may have seen considerably more activity in the early medieval period than its quiet agricultural surface now implies.