On a desolate plain in the Egyptian Sahara desert, west of Aswan, there is a very remote prehistoric site called Nabta Playa. There, a recently discovered complex of extremely ancient man-made megalithic structures have baffled the archaeologists who excavated them. An insight into the meaning and use of the megaliths led to a step-by-step sequence of discoveries, verified by measure and calculation, revealing that the megalithic architecture at Nabta Playa is a unified and detailed astrophysical map of truly astonishing accuracy, with no less than staggering implications. Written for the educated general reader, with technical appendices, the discovery of how to decipher the system of megalithic structures is reported with gripping clarity.