Many of the surface depressions in and around the Priddy Circles (Neolithic henge monuments) are shown to be natural sinkholes, not mine workings as had been assumed. The construction of Circle no. 3 involved levelling several pre-existing sinkholes, but the ground remained liable to subsidence. Unsatisfactory ground conditions may have been one of the factors that led to the abandonment of the whole ceremonial site before the Circles were completed.