This underground passage, once marked confidently on the 2nd and 3rd editions of the Ordnance Survey 6-inch maps, has since vanished from local knowledge. The cave was last documented as sitting on sloping grassland that descended southward towards a river, positioned northwest of a rocky outcrop; yet despite these clues, its precise location remains stubbornly elusive.

Souterrains like this one are amongst Ireland's most intriguing archaeological features, typically dating from the early medieval period between the 6th and 12th centuries. These underground structures, found scattered across the Irish countryside, were likely used for storage, refuge, or both by farming communities. The Ballyannan souterrain would have been constructed by digging into the earth and lining the passages with stone, creating a network of chambers and tunnels that could extend for considerable distances beneath the surface.

The documentation of this particular site comes from the comprehensive Archaeological Survey of County Donegal, compiled by Brian Lacey and his team in 1983. Their survey catalogued field antiquities spanning from the Mesolithic period through to the 17th century, preserving records of sites like this one that might otherwise be completely forgotten. Whether the Ballyannan souterrain collapsed naturally, was deliberately filled in, or simply became overgrown and lost to memory, it now exists only in these archival records; a reminder of how easily our built heritage can slip away from us.