Bulgaria has its fair share of intriguing caves, from the Devil's Throat underground waterfall to Prohodna's eyes-like openings and the Magura's prehistoric rock art. One of the most interesting of these, however, would not be of much interest to a caver as it is neither large, nor deep or covered in stalactites. This cave is interesting for its role as a sacred place in the lives of generations of people, as well as its remoteness on the Bulgarian-Turkish border.

St Marina Grotto is in the Strandzha, Bulgaria's mystic mountain that is still little known or explored, probably because most of its roads seem to have last been repaired in the 1980s, when the Communist government was trying to encourage young Bulgarians to settle in the rapidly depopulating region. Today, the mountain is even more depopulated; the result of the post-Communist economic crisis, unemployment and emigration.

Strandzha's air of solitude and isolation is hardly new. The mountain is close to major seafaring and land routes, yet its labyrinthine terrain proved too challenging for merchants, armies and empires. Since Antiquity, they have preferred to travel around the mountain rather than through it. As a result, the locals stuck to their old ways for centuries and even millennia, while the outside world changed and modernised itself beyond recognition.

This is why the Strandzha is the place where the nestinari, or firewalkers, still dance on live embers – a pagan practice now dedicated to a pair of Christian saints, Constantine and Helena.

This is also why the local spiritual life gravitates around a network of sacred springs. They are believed to be healing and, as a general rule, are located in shallow caves and grottos, usually at the bottom of deep, dark ravines. In Antiquity, the pagan inhabitants of the Strandzha venerated their nameless Great Goddess at these sites. After Christianity arrived, another female with supernatural powers took over some of them – St Marina, the patron saint of snakes, virginity, and fecundity.

Arguably the most famous sacred grotto of St Marina is near Slivarovo village, right on the border with modern Turkey. Back in the days when the current political borders were not yet drawn, people from all over the Strandzha would arrive at the sacred grotto on the day before St Marina's feast, on 17 July. There, they would pray for healing, wash themselves and drink the water dripping inside the grotto. Unmarried girls and boys would squeeze through a narrow crevice, believing this would bring them health. The pilgrims would sleep inside or around the cave, and celebrate St Marina's feast on the following day with a shared feast. They would leave their clothes at the sanctuary, believing that any illness would stay there. Then, they would leave abruptly, being careful to not look back.

No one does this at St Marina's cave anymore. The rite died out under Communism, soon after the ancient shrine found itself behind the border's barbed wire fence. Visiting St Marina’s cave was partially restored in the 1990s, when a path was built to the site, in a bid to revive local life and to attract tourists.

The groups of pilgrims who visit St Marina's cave today perform a set of rituals better suited to a trip to a spiritual shopping centre. They fill with healing water as many plastic bottles as they can, collect the supposedly health-bringing "stone pearls" that form naturally in the rock basins under the dripping water, tie a scrap of clothing to a tree, light a candle on the small altar dedicated to St Marina, and, of course, they take photos. Then they leave, satisfied that they have added something rather exotic to their list of spiritual experiences.

However, if you visit alone, you will easily succumb to the feeling that you have entered an otherworldly space, one where old spirits still lurk amid the greenery and you wonder if they harm or help the mortals who have invaded their realm. The perpetual shadows in the deep ravine, the drip-drip of the water, the moisture-laden atmosphere and the realisation that you are far from modern civilization will make you look nervously around, expecting to see some ancient apparition.

There are two ways to visit St Marina's cave: by walking for 12 kms from Slivarovo village or with a 4WD from Kosti village. You can rent a vehicle through the Tourist Information Centre at Malko Tarnovo. In both cases, you need to register with the Border Police in Malko Tarnovo.