When the author Thomas Hardy was writing Tess of the D’Urbervilles in 1891, he chose to set the novel’s dramatic conclusion at Stonehenge, where Tess sleeps on one of the stones the night before she is arrested for murder.

What the author did not know, as he wrote in the study of his home, Max Gate in Dorchester, was that he was sitting right in the heart of a large henge-like enclosure that was even older than the famous monument on Salisbury Plain.

Though invisible at ground level after millennia of ploughing, the enclosure still survives under Hardy’s garden. It has now been given protection by the government as a scheduled monument, recognising its status as a nationally important site...