Courtesy Wikipedia: "Göbekli Tepe is an archaeological site at the top of a mountain ridge in the Southeastern Anatolia Region of Turkey, approximately 6 km northeast of the town of Şanlıurfa. The tell has a height of 15 m and is about 300 m in diameter. It is approximately 760 m above sea level. It has been excavated by a German archaeological team that was under the direction of Klaus Schmidt from 1996 until his death in 2014. The tell includes two phases of ritual use dating back to the 10th-8th millennium BCE. During the first phase (Pre-Pottery Neolithic A (PPNA)), circles of massive T-shaped stone pillars were erected. More than 200 pillars in about 20 circles are currently known through geophysical surveys. Each pillar has a height of up to 6 m and a weight of up to 20 tons. They are fitted into sockets that were hewn out of the bedrock. In the second phase (Pre-pottery Neolithic B (PPNB)), the erected pillars are smaller and stood in rectangular rooms with floors of polished lime. The site was abandoned after the PPNB-period. Younger structures date to classical times. The function of the structures is not yet clear."