The building has some very interesting construction details.
The Naryn-Kala citadel in Russia, in the area known as Derbent, has long been a location of archaeological interest. There are claims that it is the oldest city in all of Russia, and as such the locale has many historical artifacts and buildings. Now a cross-shaped building within the citadel might be among the oldest Christian churches in the world.
The building has been buried for a long time, the interior obscured from the ground level. The fortress citadel was built around 300AD and the now-underground building was discovered using the celestial phenomenon of cosmic rays. Buried 36 feet underground, the UNESCO World Heritage Site cannot easily be excavated without damaging the culturally important area. The building, which once had a dome on top, is now being studied via muon radiography method. The same type of method has been used to study ancient Egyptian tombs as well, all while doing very little in the way of damage to the area.
The process was headed up by Natalia Polukhina who has been affiliated with both CERN and with the Russian Academy of Sciences. This particular study was a cooperative effort between the MISIS National University of Science and Technology with researchers from several other organizations.
The muon radiography (or tomography) method uses cosmic ray particles (known as muons) to create 3D images of spaces that cannot be entered or excavated. Much like a CT scan, this process produces an in-depth image of the site to be studied. The cross-shaped building was found to be aligned with the cardinal directions and is made from limestone. Since muon particles decay when traveling through objects or materials, the technique can be used to measure the density of building materials and to outline where the building begins and ends by measuring distribution of muons that have been aimed at the walls and ceiling.
Inside the cross-structure, the ceilings are elegantly vaulted. In combination with the wire frame from the former dome, which still remains on site, the impression is one of great majesty and intricate construction.
The structure was at first believed to have been either a water reservoir, a Zoroastrian fire temple, or a Christian church. For two months the detectors remained inside the cross and later results were studied for nuclear emulsions. But, in order to figure out exactly how the building was constructed, the outside of the building will also need to be studied in the same manner.
So far the best guess as to what it was used for is that it was once an early Christian church. It certainly does seem very elaborate to have simply been used to store water.
Some of the oldest Christian churches in Russia date back to the 9th and 10th centuries, while in parts of Africa and Egypt the oldest Coptic churches were built in the 2nd and 3rd centuries. Estimates for when the cross-shaped building in historic Derbent was constructed put the construction date at either in the 5th or 6th century. The building is suspected to have been the highest point of the citadel during construction, having been buried at a later date.
Reference to a reservoir in the area came in the 1700s, so there is a possibility that the site may have been repurposed long after construction. The area will need to be studied in more detail to answer some of these questions and to more conclusively speculate on what the original purpose of the cross-shaped building actually was.