A 4,000-year-old timber henge lies beneath central Dorchester, including a shopping arcade and supermarket car park. Archaeologists uncovered it during construction work in the 1980s; its palisade is estimated to have stretched about 400m around much of the town centre.
Joe Doak, vice chair of Dorchester Civic Society, said 21 postholes were found at the development, with more identified on adjoining sites. Red circles about 1m across in the multi-storey car park basement mark where the palisade passed.
Doak said charcoal and burnt material in the pits suggest the structure burned down. The reason is unknown; he said it may have been a celebration or part of a conflict.
