Turkey, which was originally a part of ancient Mesopotamia, has long been a haven for archaeologists. It is home to important structures that even precede Mesopotamia, including UNESCO World Heritage structures like Catalhoyuk, a proto-city that dates back around 9,000 years, and Gobekli Tepe, a Neolithic village said to be more than 10,000 years old with what may be the world's oldest seat of worship.

Ruins of housing believed to have been used by soldiers stationed at the garrison.

The footprint of pre-Roman activity in the region is now being moved farther east than was previously thought thanks to recent discoveries at sites in the southeast of the country, according to archaeologists.

The Roman Empire's military fortress at Zerzevan Castle is supplying what UNESCO refers to as "important information about the Roman soldiers, civilians' daily lives and the battles."

A local family visiting Zerzevan Castle archaeological site.

The Mithras Temple is still another option. The Mithras religion, commonly referred to as the "Mithras cult," is thought to have started in ancient Persia, and the temple, found in 2017, may be the best-preserved example of this type of temple anywhere in the globe, according to UNESCO.

Large, multistory structures have been discovered by archaeologists using ground-penetrating radar scans, although they have not yet been excavated. These are still underground and are uncovering reams of relics, some of which go all the way back to pre-Roman times.

Archaeologist Aytac Coskun claims that when he first saw the location, he knew he had to conduct excavations. The ancient church was constructed on a hill high above the temple.

Archaeologist Aytac Coskun, seen at ruins of an ancient church, says excavations in the area may continue for another three decades.

"I first came to Diyarbakir in 2005," recalls Coskun, "and when I saw this hill, I saw some pieces of artifacts, and I knew no excavation had been done before. So as soon as I saw it, I knew it had to be a dig because there must be something significant underneath."

During the conflict, 10,000 people may have found safety in underground residential areas

A tour of the site displays some of the recent discoveries made by him and his team, including an expansive rock altar, a subterranean church, and a water canal that extends for at least a few miles.

A member of the Zerzevan Castle excavation and restoration team looks into a microscope while studying a coin found at the archaeological site.

Coskun and his crew have discovered a number of artifacts, including a superbly preserved and elaborately painted bronze baptismal pail from the Roman era and a possible 3,000-year-old Assyrian stamp cut into rock.

"The digging we're doing inside the castle walls is 57,000 square meters [68,171 square yards]," he mentions. "It's a huge area. And outside of it...is (something) like 10 million square meters [3.86 square miles]."

A bronze baptismal bucket found at the Zerzevan Castle site, currently on display at the Archaeology Museum of Diyarbakir.

Coskun estimates that during periods of peace, 1,500 people—military and civilian—lived here. He asserts that it is likely that 10,000 residents of the neighborhood fled here during a time of conflict to find refuge.

That, he claims, might assist in explaining the huge underground living spaces. He claims that there are still 99 dwelling complexes below the surface in addition to the six that have already been discovered inside the castle walls.

An inscription seen at the entrance of the Mithras temple which remains undeciphered.

Coskun cites this as just one example of how this location may alter how people now view this region of the world and its archaeological and architectural past.

Ruins of the church seen from the south tower.

"It's totally open to new discoveries, that's for sure," he says. "We don't know what else we'll find. We've only dug around 10% of the area on the surface within the castle walls. And beyond the castle walls," he continues, "you see more living areas, the canal, a necropolis where the leading families buried their dead, and ceremonial areas. So, there will be more to come."

He estimates that excavations might go on for another 30 years.