The path down to the grotto drops steeply through oak and hornbeam. By the time you reach the ravine floor the sky has almost disappeared. The light here is greenish and perpetual, unchanged whether it is morning or afternoon, July or October. Water drips somewhere ahead. The air smells of wet stone and something older – soil, perhaps, or time itself.

This is St Marina's Grotto near Slivarovo, in the deepest folds of the Strandzha, Bulgaria's southernmost mountain range. Remote, barely visited, and straddling the Bulgarian-Turkish border, it is one of those places that makes the word "ancient" feel inadequate.

The Strandzha has always resisted the outside world. Close to major seafaring and land routes, its labyrinthine terrain proved too challenging for merchants, armies and empires since Antiquity. They preferred to travel around the mountain rather than through it. That isolation preserved something rare: a way of life, and a way of believing, that endured for centuries while the rest of the world modernised around it. This is why the Strandzha is the place where the nestinari, or firewalkers, still dance on live embers – a pagan rite now dedicated to the Christian saints Constantine and Helena, the two strands of belief wound so tightly together that no one can say where one ends and the other begins.

It is also why the spiritual life of the mountain gravitates around a network of sacred springs, nearly all of them located in shallow caves and grottos at the bottom of dark ravines. In Antiquity, the pagan inhabitants of the Strandzha, the ancient Thracians, venerated their nameless Great Goddess at these sites. When Christianity arrived, another female figure with supernatural powers took over many of them – St Marina, the patron saint of snakes, virginity and fecundity.

The grotto near Slivarovo is arguably the most famous of St Marina's sacred sites in the Strandzha. In the days before the current political borders were drawn, pilgrims arrived from all across the mountain on the eve of the saint's feast, 17 July. There, they prayed for healing, washed themselves and drank the water that drips inside the cave. Unmarried girls and boys squeezed through a narrow crevice in the rock, believing the passage would bring them health. The pilgrims slept inside or around the cave, celebrated with a shared feast the following day, left clothing at the sanctuary so that any illness might stay behind with it and then departed abruptly, taking care not to look back.

All of this stopped under Communism, when the ancient shrine found itself behind the border's barbed wire fence. The rite was never properly revived. Since the 1990s, when a path was cut to the site in an attempt to attract visitors and breathe life back into a depopulated region, a different kind of pilgrimage has taken its place. Today's visitors arrive in groups, fill plastic bottles with the healing water, collect the small "stone pearls" that form naturally in the rock basins under the drip, tie a scrap of cloth to a tree near the entrance, light a candle on the modest altar and photograph everything carefully. Then they leave, the ritual complete.

Come alone, though, and the place does something different to you. The perpetual shadows, the sound of water, the realisation of how far you are from anything ordinary – all of it conspires to make the rational mind loosen its grip. The cave feels inhabited, though by what exactly is hard to say. Old spirits, perhaps, the kind that have been accumulating here since long before the saint's name was attached to the rock. Whether they harm or help the people who enter their realm is a question the Strandzha has never seen fit to answer.

Getting there

St Marina's Grotto can be reached either on foot from Slivarovo village (approximately 12 km) or by 4WD from Kosti village. Vehicle hire can be arranged through the Tourist Information Centre in Malko Tarnovo. Visitors must register with the Border Police in Malko Tarnovo before setting out.