The archaeological site in Anatolia, Turkey, called Göbekli Tepe (map) is now a UNESCO World Heritage site. It came to the attention of archaeological scholars in 1963, and is estimated to date from around 12,000 years ago. Its construction is reckoned to have continued over a period of more then 1,000 years.

Its functions are unknown, though it's generally assumed to be principally associated with religious practices. No residential buildings or fortifications have been discovered so far.

The building process - bearing in mind that some of the stone blocks are estimated to weigh up to 70 tons - is also unknown.

Before the invention of pottery, agriculture, domesticated animals or writing, hunter-gatherers constructed this vast ceremonial site of at least fifty colossal T-shaped pillars, some as high as 17 feet, adorned with intricate reliefs of totem animals such as jaguars, symbols of death like carrion birds, and game animals such as the wild boar that still roam these hills today. Covering the entire hillside and visible for miles around in its day, these Early Neolithic monoliths boast diameters ranging from 30 to 100 feet, weigh up to 20 tons and were each set within around 20 concentric rings, the widest measuring 30 yards across."

Source : Global Heritage Fund

To date, it's estimated that only around 5% of the site has been excavated.

More details from the German Archaeological Institute ^{[mostly in German]} which is still excavating the site.