Every August, followers of the Universal White Brotherhood converge from across the world on the Rila lakes, at an altitude of 2,100 metres, to celebrate the beginning of their new year at the height of summer.
The highlight of the day is "paneurhythmy", a dance-like ritual that pilgrims perform in large concentric circles, creating a striking image on the verdant mountain plain.
The esoteric society combining Christianity and Indian mysticism believes that positive cosmic energy is at its strongest here around August 19 and that paneurhythmy will help them channel it and spread it around the world.
The pilgrims, who address each other as "brother" and "sister", camp near the Seven Rila Lakes, greeting the sunrise with prayers and meditation before hiking up to the nearby plateau to practise paneurhythmy. They also attend lectures to open their souls and fill them with positive energy.
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The spiritual school, founded by Bulgarian theologian Peter Deunov in 1897, emphasises brotherly love, healthy habits, positive thinking and living in harmony with nature.
Deunov -- also known as Master Beinsa Douno -- began developing paneurhythmy and taking his followers camping to Rila in the 1930s, by which time they numbered around 40,000.
The Bulgarian government later recognised the positive health effects of his "cosmic rhythm" exercises, and paneurhythmy was experimentally taught in a Sofia school as a physical education option.
But World War II and the installation of communist rule in Bulgaria in 1944 drove the movement underground.
"Paneurhythmy was practised in secret and people still came to Rila even if it was officially banned," said 68-year-old Hristo Madjarov, one of the movement's lecturers and a long-time follower.
Authorities closely monitored meetings in Sofia, and the secret police often questioned members, Madjarov added.
Even now, the Universal White Brotherhood remains at odds with the Christian Orthodox Church, which has dismissed it as a sect.