When travelling around the Eastern Rhodope, you are bound to encounter this strange sight: on certain precipitous rocks, here and there, are scattered small, dark niches. Some are on their own, others form groups of dozens.

What are these strange niches, you might wonder. Nobody knows for sure, is the honest answer. The mystery of the rock niches that indent major cliffs in the Eastern Rhodope remains unexplained.

Less than a metre high, the niches are usually trapezoid, although you can encounter circular, rectangular or square ones. The most spectacular niches are high above ground, forcing you to crane your neck to see them, though many are found at ground level. More than 200 groups of niches have been discovered so far, with the biggest concentration in the Gluhite Kamani, Madzharovo and Valche Pole areas. Rock niches are a regular feature at rock shrines created by the ancient Thracians in the 1st millennium BC, but not all seem to belong to such places.

Rock niches near Dzhanka village

Of course, lack of information opens the door to speculation. Some scientists believe that the rock niches were made to hold the burial urns of cremated Thracians who could not afford the more expensive and prestigious rock tombs. Yet, the bases of many of the niches are angled upwards, making it impossible to put anything inside them. Others claim that the niches were hewn by adolescent Thracian boys as part of an initiation ceremony. According to a third hypothesis, the niches depict the stars and the constellations in the heavens, while a fourth proposes that they were some sort of map, indicating the whereabouts of ancient gold mines. According to yet another idea, the niches are scaled-down models of dolmens and rock tombs or, more interestingly, of the cave symbolising the womb of the Great Goddess.

Here is another theory: the niches were symbolic doors into the realm of the Great Goddess and her son and lover, the Great God, who, according to some historians, were the Thracians' main deities. The great majority of niches look to the south and east, maybe suggesting a sun cult, but many face north and west and are in places that rarely see the sun, including caves, to make such an interpretation credible.

Orlovi Skali, or Eagles Rocks, near Ardino

Whatever the explanation, the presence of these niches at rock shrines, together with other carvings such as rock tombs, rock stairs and rock altars, indicate some ritual or religious activity.

Their geographical location is also curious. Rock niches are concentrated in the Eastern Rhodope along the course of the River Arda. They are rare in the rest of Bulgaria, but some researchers find some parallels to (much bigger and better shaped) niches in what used to be the ancient land of Phrygia, now in western Asia Minor in Turkey.

How the niches were created is another open question. An experiment with tools from the period has shown that it would take a day to carve a single niche into the rock. Those high above the ground might have been created by people standing on scaffolding, rather than hanging on ropes. In certain groups, the shapes of some niches look so similar that they might have been made by the same person.

The niches near Dazhdovnitsa village were cut as high as 45 meters above ground. They are among the highest rock niches discovered in Bulgaria so far

Whatever their purpose or construction method, by the 5th century BC the Thracians stopped making niches – in the same way they stopped carving rock tombs. Why the change? No one can tell. The last person who knew the answer died centuries ago. What is left to modern researchers is to pick over scraps of facts and evidence, and to try to create of them a coherent picture of the past.

Niches dot the rock shrine at Gluhite Kamani, or Deaf Rocks. The sanctuary also includes two tombs, a staircase and a water cistern cut in the rocks, together with some temple buildings 

Rock niches and tombs dot the environs of Dolno Cherkovishte village

Rock niches near Nochevo village