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Cosmic ray exploration sounds like something out of a sci-fi movie and muon radiography just sounds made up. But by using these very real technologies, researchers have discovered something much more connected to our history than to our potentially extraterrestrial future: ancient ruins of what might be the world’s oldest church.

What is the situation?

In the ancient city of Derbent in southern Russia lay the remains of an unexplained structure. This structure is located on the northwest side of the Naryn-Kala fortress, which dates back to 300 A.D. The fortress was buried almost 400 years ago, and while much of it has been uncovered, there are still some hidden areas that researchers are eager to explore. The structure looks to be cross-shaped, but because the fortress itself is a protected UNESCO cultural heritage site, no one can touch it, much less dig it out. A group of researchers from the Russian Academy of Sciences, the Skobeltsyn Institute of Nuclear Physics at Lomonosov Moscow State University, and Dagestan State University decided to get to the bottom of it, literally, without disturbing anything.

How is cosmic ray exploration connected to archaeology?

Cosmic rays flow towards the Earth all day, every day from an unknown source in the solar system. The majority of these rays are easily blocked by our protective atmosphere, but special particles from these rays, called muons, actually make it through to the ground. Scientists have created a way to use these muon particles to their advantage. Since muons lose energy and die when they move through dense objects, it’s possible to determine the approximate density of an unknown, unseen object by tracking the number of muons at different places. If there is a distinct density difference between an underground structure and the surrounding soil, for example, scientists can use muon tests to determine the size and shape of the buried structure. When researchers in Russia were told they would be unable to physically excavate and explore a remarkable ancient church, that’s exactly what they did.

Is it a church?

Ever since this structure was discovered people have been speculating about its original uses, particularly because it is almost entirely buried. The top of the structure sticks out of the ground a little bit, but it continues down into the Earth nearly 36 feet (11 meters). For centuries, various sources have referred to this particular structure as a water tank, but after using muon radiography at the site, researchers are confident that that is not the structure’s original intent. After testing and measuring the site for a couple of months, some key clues to the structure’s purpose became clear. First, there is a rectangular box-like structure within the larger space that may have been a water tank, so it is unlikely that the entire structure was used to hold water (though that is what people in the 17th and 18th century may have done). There is also a much larger water tank elsewhere in the fortress. Additionally, it is now clear that this structure once was the highest part of the overall fortress. The theory that this structure once was a church stems from the cross shape and its prominent position at the top of the fortress. Researchers are still unsure what the exact purpose of this structure was, but they are excited to keep exploring, especially with new technologies like using muon rays.

For now, all we can do is make predictions about what ancient peoples intended for this structure. Meanwhile, researchers all over the world have started using this type of non-invasive investigative technologies to discover and learn more about our world’s buried treasures.