This is a souterrain, an early medieval underground structure cut from the earth and used, most likely, for storage or refuge, and the one at Ahakeera is notable for its scale and relative completeness: five chambers connected by low creepways, the whole system stretching in a chain beneath the ground.

The site came to light in 1973, and was documented in detail by Twohig in 1976. The five earth-cut chambers vary in size but share a consistently low height of around one metre, meaning access would always have required crouching or crawling. The largest chamber measures 4.2 metres long and 2.1 metres wide; the smallest recorded is 2.3 metres long. Two of the connecting creepways were built with jambs, upright stones framing the passage openings, a feature that could have supported a removable block or door. Construction shafts above chambers one, two, and three suggest how the builders worked: digging down from the surface to excavate each chamber before covering them over again. A stone-covered drain runs from chamber four, indicating that water management was built into the design from the start, and the probable original entrance appears to have been located in chamber one. Together these details point to a carefully engineered structure rather than a simple pit, one that was meant to function efficiently and remain concealed.