That is one of the more arresting qualities of a souterrain, an early medieval underground passage or chamber system, usually stone-built, whose purposes likely included food storage, refuge, and concealment. The one at Ballyellis had been there all along, waiting beneath the soil with no outward sign of its existence.

When the site was investigated following its accidental discovery, at least two chambers were recorded, connected by a narrow creepway, the low linking passage through which a person would have to crawl. The first chamber is roughly circular, measuring around 2.7 metres east to west and 2.8 metres north to south, with a maximum height of 1.75 metres, enough to stand in, just. A creepway just 1.2 metres long and 0.6 metres high leads westward from it into the second chamber, which is similarly circular in plan and nearly identical in width but considerably lower, reaching only 1.3 metres at its highest point. A second creepway leads off from the south side of this chamber, built of drystone walling and roofed with lintels, though it was found blocked by loose earth and stones. Whether that passage connects to a third chamber or simply terminates has not been established. The whole structure is dry-stone construction, roofed with flat lintels, a building technique that required considerable skill and planning, and one that has kept these chambers intact underground for well over a thousand years.