At just four and a half metres across, it is easy to miss entirely, and for a long time it was missed, only coming to light when an aerial photograph revealed the tell-tale shadow of a ring-ditch, the circular trench that typically surrounds a prehistoric barrow or low burial mound.

A ring-ditch of this kind is essentially what remains after a barrow, a mounded earthen grave, has been levelled by centuries of agriculture and weathering, leaving only the encircling fosse, or ditch, as evidence that something once stood here. At Lissobihane, that fosse is shallow, just thirteen centimetres deep and roughly a metre and a third wide, and its south-western and north-western sections hold standing water, sitting as they do just above the level of old drainage channels in the surrounding land. The interior of the circle has settled to roughly the same height as the ground outside it, which suggests whatever mound once occupied the centre has long since been worn flat. The whole feature measures the same distance north to south as it does east to west, a near-perfect circle, which is itself characteristic of deliberate prehistoric construction rather than any natural formation.