CIMRM Supplement - Mithraeum. Zerzevan Castle, Diyarbekir, Turkey.

In 2017 news reports have appeared of the discovery of a temple of Mithras under Zerzevan Castle (Zerzevan Kalesi), near Diyarbekir. This from the Daily Sabah:

During excavations at the Zerzevan Castle in Diyarbakır's Çınar district, a 1,700-year-old Roman-era underground temple belonging to the Mithras religion has been discovered.
Excavations near the Demirölçek neighborhood, located 13 kilometers from the Çınar district, have been ongoing since 2014 with the cooperation of Turkey's Ministry of Culture and Tourism, the Diyarbakır Museum, the Diyarbakır governorship, the Çınar district governorship and Dicle University.
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The head of the excavations, assistant professor Aytaç Coşkun, a faculty member of the Department of Archaeology at Dicle University, noted that they believe the temple is the only Mithras temple on the eastern border of the Roman Empire and is, therefore, of crucial importance. He also added that the religion of Mithras was a religion of mystery widely popular among Roman soldiers.
"The followers of this religion are from a closed community because their religious ceremonies are completely secret and no information was leaked to outsiders. Mithras represents both the sun god and also "consensus." Their temples are usually built underground. There are three niches on the eastern part of the temple. A very thoroughly constructed one is in the water basin. There is also a pool. We believe water was very widely used in Mithras ceremonies and about 40 people attended ceremonies held here," Coşkun said.
With the arrival of Christianity, the religion of Mithras lost its importance and the recently discovered temple is from the peak times of this religion.
Coşkun noted that the temple is 35 meters wide and reaches a height of 2.5 meters. He added that because these such temple were built underground, the temples are usually not large structures.
"The excavations at the castle are important for shedding light on the ancient Roman area and will have a tremendous impact on the history and tourism in the region," Coşkun said.
He added that the ruins have been preserved without much damage and the castle is a city of its own both under and above the ground.
The manager of City Culture and Tourism Cemil Alp said they obtained an enormous amount of information on the history of the region and that he believes there will be a breakthrough in tourism with the exhibition of the artifacts found.
Zerzevan Castle
The Zerzevan Castle is situated along the ancient route of military premises and located on a 124-meter-high rocky hill in a strategic location between Amida and Dara. The settlement overlooks the entire valley and once controlled a large area on a key, ancient trade path. Once a strategic Roman border garrison town, the castle also witnessed the clashes between Romans and Sassanians.
The first settlement was named "Samachi" and while it is not certain when it was built, the excavations are close to revealing its age. The castle walls were repaired at the time of Anastasios (491-518 A.D.) and Justinianos (527-565 A.D.) and some parts have been completely reconstructed.

More info from here, via Google Translate:

Just north of the shelter, the building was constructed by engraving the main rock in the ground, 7 meters in length, 5 meters in width, 2.5 meters in height and 35 meters in length. On the eastern wall of the structure there are pillars carved into the main rock and two large niches in the middle and two small niches in the middle. Also the beam of the eastern wall of Mitras, the crown motif, is also etched. In one of the smaller niches is a water carved fairly neatly and there is a swimming pool right in front of it. It is connected with a canal and a canal passing through the pool wall, and it is known that water was used in the Mithras religious ceremonies. There are also four symmetrical animal mounts in the ceiling. Linking places must have been used to sacrifice the bull in Mithras rituals. The building is the first and only Mithras temple to be found on the eastern border of the Roman Empire. The inscription and the symbols are clearly visible at the entrance gate of Yapi. It was transformed into a church with the spread of Christianity, and Zerzevan Castle was used as shelter after being abandoned.