"When we open a new trench, we never know what to expect," said Lee Clare of the German Archaeology Institute, who has been excavating there since 2013.

"It is always a big surprise."

Gobekli Tepe, which means "Potbelly Hill" in Turkish, is arguably the most important archaeological site on Earth.

Thousands of our prehistoric ancestors gathered around its highly-decorated T-shaped megalith pillars to worship more 7,000 years before Stonehenge or the earliest Egyptian pyramids.

"Its significance is hard to overstate," Sean Lawrence, assistant professor of history at West Virginia University, told AFP.

Academics believe the history of human settlement began in these hills close to the Syrian border some 12,000 years ago when groups of Stone Age hunter gatherers came together to construct these sites.

Gobekli Tepe—which some experts believe was never actually inhabited—may be part of a vast sacred landscape that encompasses other nearby hilltop sites that archaeologists believe may be even older.

Endless mystery

None of which anyone would have guessed before the German archeologist and pre-historian Klaus Schmidt began to bring the first discoveries to the surface in 1995.