Police checkpoints, scores of cars parked along the roadside and throngs of people crowding between stalls selling candyfloss, kepabcheta and cheap Made-in-China toys: on 3 June, the village of Balgari looks much like any Bulgarian village during a country fair.

Balgari's fair, however, is unlike any other. When darkness falls over the village square, barefoot men and women will dance on live coals.

Nestinari dances are a ritual particular to the Strandzha mountains, a relic of a time that has survived because of the region's centuries-long isolation, far removed from major routes and large cities. Many villages, some populated by Greeks and others by Bulgarians, used to have nestinari, but today Balgari is the only place where the genuine rite survives in Bulgaria. In Greece there is now only Langadas village near Thessaloniki.

Once, the ritual was a regional affair enacted far from the curious eyes of outsiders. Today, its rarity, mystery, and role in the Bulgarian identity attract a public eager to experience something exciting, patriotic and YouTube-friendly. In the past decade, the number of Bulgarians who flock from far and wide to Balgari to witness the nestinari dancing on live coals has increased exponentially.

The 3 June event, which falls on the feast day of Ss Constantine and Elena according to the Julian Calendar, unfolds in a special manner. In the morning, nestinari, or the men and women who will dance on the live coals, gather in their special chapel. They take the saints' icons and carry them to a holy spring where they bathe and "dress" them, as if they were real people, in new kerchiefs. Meanwhile, a bonfire is made in the village square.

When dusk falls, the expectant crowd gathers around the circle of hot embers the bonfire has burnt down to. Back in the chapel, the nestinari slowly enter into a trance. When they are ready, they leave for the square, preceded by a bagpiper and a drummer beating the special nestinari drum, which is kept in the chapel all year round and only taken out for the 3 June ritual. The tune is also special, its rhythm seemingly mesmerising both the nestinari and the crowd.

Once the nestinari reach the square, they process three times around the church of Ss Constantine and Elena. These saints, particularly Constantine, are the patrons of the rite.

Icons of Ss Constantine and Elena kept at a special nestinari chapel at Balgari

Then the nestinari return to the embers that glow in the dark. The music continues. One by one, icons of Ss Constantine and Elena held in their hands, the dancers step onto the live coals. They cross the fire, weaving in and out until, almost imperceptibly, all step out and, bare feet seemingly untouched by the heat, they head back to their chapel, while the cooling coals slowly turn into ash.

The origin and meaning of the nestinari dances is one of the Balkans' greatest mysteries. Since they were recorded for the first time, in the 1870s, when the ethnically mixed region was within the Ottoman Empire, countless Bulgarian and Greek priests, historians and anthropologists have tried to crack the nestinari code, to no avail.

This is what we do know so far.

In former times, nestinari dances were performed not only for the feast day of Ss Constantine and Elena but also on the feasts of St Marina, St Elijah, St John the Baptist and even at Easter. The Greek-inhabited villages Kosti and Brodilovo, and the Bulgarian Urgari (today's Balgari), were at the centre of the practice, but others villages also celebrated the rite.

Dancing on live coals was just one of the many things the nestinari did. The local community held them in high esteem, for they were wise, experienced healers and unerring fortune-tellers, leading virtuous and even ascetic lives. There would be one main nestinar, often an old woman, universally venerated for their wisdom; the moral compass of the community.

Organised into loose societies with their secrets, the nestinari preferred their own chapels to village churches, and rarely paid their respects to established Christian feast days. However, local priests tolerated them and would even lead the procession to and from the circle of live embers.

Most of the time the nestinari led a normal life. Their behaviour would start to change with the approach of the feast day of the patron saint. Then they would be "possessed" by the saint's spirit. They would shake, turn cold and pale, scream and enter into a trance. Some claimed they could hear St Constantine talking to them. While dancing on the embers, they would predict the future.

Some families were more likely to produce nestinari, but anyone, particularly young women, could become possessed. Stopping someone from entering a trance and dancing on the embers could result in great psychological distress and even fatal illness.

This is why, according to some analyses, nestinari were a primitive form of individual and group psychotherapy.

No nestinari would enter the live coals without the precious protection of an icon of Ss Constantine and Elena

Nestinari dedicate their dances to Christian saints but the origin of the rite is clearly pagan, though which deities it was devoted to remains a mystery. No such rite was recorded in the region before the 1870s, and its closest analogues from Antiquity are from Italy and Cappadocia, in Asia Minor.

Nowadays, most Bulgarian historians think that the nestinari rites are a Christianised version of Dionysian festivities, or of ancient Thracian rites dedicated to the nameless Great God and Great Goddess, who created and ruled the world.

The problem with studying nestinari properly is that when the rite was still alive, in the late 19th and early 20th centuries, practitioners were secretive about their beliefs, rituals and experiences. Anthropologists were rarely able to witness nestinari in action, and had to rely on accounts from other witnesses.

In the early 20th century, another hurdle appeared: modern life finally reached the Strandzha. Afraid of appearing old-fashioned and uneducated to outsiders, the locals preferred to keep their mouths shut rather than share information about the rite. The nestinari themselves adapted to the new times and started to lose the most "embarrassing" part of the ritual: the trance and the firewalking. They reinvented themselves as healers and community leaders.

International politics then intervened. After the Great War, Bulgaria, Greece and Turkey switched populations. The inhabitants of the old Greek nestinari villages left for good, taking the ritual with them. This was how anastenaria dances appeared in Langadas.

At the same time, the Bulgarian Orthodox Church denounced the already dying ritual as a pagan abomination.

The final blow came with the Communist coup in Bulgaria, in 1944. The new, atheist government favoured building a new, modern society over the preservation of mystic and "backward" traditions. It labeled official religion and folklore rites, such as nestinarstvo, as "dark superstition" and actively discouraged them.

At their chapel, nestinari prepare to go into trance

When Communist anthropologists began to study the ritual in the 1950s and the 1960s, from a Marxist point of view, only a handful of elderly people practiced nestinarstvo. The long chain of tradition had been severed and scientists and the public focused on the most spectacular part of the rite: the firewalking. Perceived as solely a physical feat, it became a tourist attraction. In the 1970s, performers started dancing on coals at restaurants in the Bulgarian Black Sea resorts, and some traditional restaurants across Bulgaria offered such shows as late as the 2010s.

In the 1970s, the nestinari rite had not only been turned into a spectacle for tourists, but a new line of historical thought was developed in another direction. It overstressed the supposed connection of the rite to the ancient Thracians, who were being overhyped as one of the most spiritual and wise of ancient nations, and even the progenitors of Greek civilisation. As Bulgarians were seen as partially descended from the Thracians, nestinari were interpreted as key evidence of the nation's millennia-long history and spirituality. This was a crucial tool for the Communist regime, which at the time was turning increasingly nationalistic in order to strengthen its grip over the nation and to provide some distraction from the struggling economy.

When Communism collapsed, in 1989, the nestinari rite revived in the Strandzha. Regular meetings with Greek anastenaroi from Langadas are now organised, and in 2009, the rite became a UNESCO Intangible Cultural Heritage of Humanity.

The fire at the centre of Balgari burns for a whole day in order to be ready for the nestinari

Post-1989, the trend of turning the rite into a patriotic crowd-puller intensified. Today, Bulgarians are proud of the physical endurance of their nestinari, and of the centuries-long tradition. However, for the general public the rite begins and ends with firewalking. The diverse traditions and rites performed by the old nestinari have been forgotten and/or simplified to make them more palatable to the modern mind. In their attempts to attract more tourists to the economically-struggling Strandzha, local authorities have turned the 3 June Balgari event into a general spectacle. It is not only the crowds with their smartphone cameras that spoil the atmosphere, as before the firedancing begins, folk music atypical to the Strandzha region blasts out over the village square, and a huge LCD screen broadcasts the event from start to finish.

The nestinari themselves have also changed. Unlike a century ago, when their rituals were off-limits to outsiders, today you can visit them in their chapel while they prepare to enter the trance. They will even tolerate you taking photos there, particularly if you offer them a small fee.