The hollow, roughly fifteen feet long, eight feet wide, and two feet deep, is the only visible trace of an underground structure that once gave the fort its very name.

A souterrain is a dry-stone underground passage or chamber, typically built during the early medieval period and associated with ringforts; they were used variously for storage, refuge, or concealment. The one at Derryleagh was apparently of an unusual form. Around 1980, local accounts describe two beehive-shaped stone chambers, connected by a low crawlway known as a creepway, being exposed, presumably through accidental disturbance or informal investigation. They were subsequently infilled, leaving the site as it had been: a quiet dip in the earth. The connection between the souterrain and the fort's name was already noted in 1934 by a writer named Bowman, who recorded the depression and identified it as the location of the cave that gave the enclosure its identity. That the underground chambers went back underground, sealed once more, gives the place a quality that is more than simply archaeological; something was found, recognised, and returned to the dark.