The three hollows, running roughly north to south across a span of about eleven metres, are where the ground above a souterrain has slowly subsided over the centuries. A souterrain is an artificially constructed underground passage or chamber, built during the early medieval period, typically beneath or alongside a rath. A rath, also called a ringfort, is a circular earthwork enclosure that served as a defended farmstead; they are common across Ireland, and this example at Killeenrevagh sits within one.

The passage itself can be accessed through an opening at the southern end of the middle depression. It is 1.4 metres wide, built with drystone walls and roofed with flat lintels laid across the top, a construction technique typical of the type. From the opening, the floor slopes downward for a few metres, partly obstructed by accumulated soil and loose stones, before levelling out at the southern end where there is enough headroom to stand upright. The passage runs for six to seven metres before meeting a blank wall. That might seem like the full extent of it, but the landowner knows otherwise. At the southern end of the western wall, a low narrow opening, now blocked, once connected to a second passage beyond. Whether that further section survives intact underground is unknown, and the blocked opening gives no indication of what lies on the other side.

The northernmost of the three surface depressions is the shallowest, and a small opening, possibly lintelled, is just visible at its southern edge. This suggests the underground structure extends further north than the accessible passage alone, adding a further layer of uncertainty to what is already a partly legible, partly obscured piece of early medieval engineering. The subsidence pattern overhead and the blocked side passage together make this a site where most of the story remains, quite literally, out of reach.