The huge stone circles built in the British Isles and northwest France from 6,500 to 3,500 years ago are among civilization's strangest monuments. Ignored or plundered for centuries, they have only in modern times begun to reveal their remarkable complexity. It is now widely acknowledged that these ancient sites were precisely aligned to major celestial events, probably linked to the agricultural calendar of early farming settlements. But the mystery remains as to how the megalithic builders achieved such extraordinary accuracy in their measurement. Inspired by the surveying work of Alexander Thom, Anne Macaulay devoted her life to an investigation of the stone circle sites, seeking out their hidden geometry and deeper cultural significance. In this book, she draws on ideas from geometry and metrology, archaeology and anthropology, history and mythology, astronomy and music. Macaulay concludes that the extraordinary mathematical skills of the British megalithic builders was original and self-contained, but that in turn, the elite of this society became the proto-Greeks, their knowledge flowing to the Eastern Mediterranean.