The collapse was spontaneous, nobody dug here, and the opening it left, roughly 1.6 metres by 0.83 metres, became an accidental window into a souterrain that had been sealed from the world for centuries. A souterrain is an underground structure, typically early medieval in date, built or cut as a place of refuge, storage, or concealment, and this one had kept its secrets well.

The souterrain at Knockane is earth-cut rather than stone-lined, meaning its chambers were hollowed directly from the soil rather than constructed with masonry, which makes the detail of its survival all the more striking. At least four chambers have been identified, connected by creepways, the low, narrow passages that force anyone moving between chambers to crawl. Chamber 1, just under a metre below the surface, is oval in plan and accessible only through the collapsed roof. From it, three separate creepways branch outward in different directions, north, east, and south-south-west. Most lead into spaces that remain blocked by fallen earth and stone, inaccessible to investigation. One small detail stands out: a narrow opening, barely 23 centimetres wide and 31 centimetres high, sits just above floor level between chambers 1 and 2, functioning as a kind of interior window that allowed people in adjacent chambers to see or communicate with one another without passing through. Chamber 4, the largest mapped so far at roughly 6 metres by 4 metres, was not entered, and a creepway in its north-east corner suggests the network may extend further still.

The site sits in open pasture at the top of a hill with views in all directions, which is itself a reminder that souterrains were rarely built in isolation. They were typically associated with early medieval settlement, and a commanding hilltop position would have suited both habitation and the kind of watchfulness that made underground retreats worth constructing in the first place. Much of the interior remains unexcavated and blocked, so the full extent of what lies beneath Knockane is still, in a literal sense, unknown.