Prohodna Cave (Eyes of God)

Prohodna Cave, locally known as Prohodna or 'The Eyes of God' (Ochi na Boga), is a passthrough cave and Natural Monument located near the village of Karlukovo in Lovech Province, in the karst region of the northern Balkan Mountains. The cave is distinguished by two large eye-shaped openings in its roof that allow natural light to stream into the approximately 262-meter-long passage, creating a dramatic illumination effect that gives the cave its popular name. Prohodna is the longest passthrough cave in Bulgaria and one of the most visited natural attractions in the country, drawing tens of thousands of visitors annually. The cave is accessible without specialized equipment and is particularly popular with hikers and tourists exploring the Iskar Gorge area.

Prohodna Cave serves as an important roost for several bat species, which use both the main passage and the twilight zones near the entrances. The cave's passthrough nature and dual ceiling openings create a distinctive microhabitat with zones ranging from fully illuminated to completely dark. Bat species using the cave include the Greater Horseshoe Bat and various Myotis species. The cave walls host cave-adapted invertebrates including cave spiders (Meta species), cave crickets (Troglophilus species), and various beetle species. The Iskar Gorge surrounding the cave supports a rich fauna including otters in the river, peregrine falcons and eagle owls on the cliff faces, and numerous woodland species in the surrounding forests. The gorge is recognized as an Important Bird Area.

The vegetation of the Karlukovo area and Iskar Gorge is characteristic of the limestone karst zone of northern Bulgaria. The cliff faces and rocky slopes of the gorge support thermophilous woodland dominated by pubescent oak, manna ash, and hop-hornbeam, with a rich shrub layer including smoke tree, wild privet, and box. Limestone grasslands on sunny ledges and plateau surfaces harbor a diverse calcareous flora including numerous orchid species, rock roses, and various bulbous plants. Near the cave entrances, the vegetation transitions to shade-tolerant ferns, mosses, and moisture-loving herbs. The Iskar River corridor supports riparian forests of alder, white willow, and black poplar.

Prohodna Cave is developed in the Cretaceous limestone formation of the northern Balkan Mountains. It is a passthrough cave, meaning that a stream has eroded a complete tunnel through a limestone ridge, with separate inlet and outlet entrances at different elevations. The two large ceiling openings—the 'Eyes of God'—are collapsed portions of the cave roof where the limestone above was too thin to withstand erosion and weathering. The cave passage is broad and relatively unobstructed, indicating high historical water throughput. The Iskar Gorge, which the cave overlooks, is one of the most spectacular examples of river incision in Bulgaria, cutting through the Balkan Mountains to create cliffs hundreds of meters high in places. The regional karst contains numerous caves, sinkholes, and springs.

Lovech Province and the Iskar Gorge experience a moderate continental climate with cold winters and warm summers. The gorge walls provide some shelter from winter winds, creating slightly milder conditions in the valley bottom. The cave interior maintains a constant temperature of approximately 12°C year-round with high humidity, making it noticeably cool in summer and relatively warm in winter compared to outside conditions. The cave passage can experience strong air currents when outside and inside temperatures differ significantly, with cool air draining downward from the higher entrance in summer and warm air flowing upward in winter. Annual precipitation in the region is approximately 600–700 mm. The spring period following snowmelt can produce temporary flooding of the cave floor during periods of high karst groundwater.

The Iskar Gorge region has been inhabited since prehistoric times, with the limestone caves providing natural shelters for ancient peoples. The Bulgarian medieval fortress of Karlukovo Monastery and other ecclesiastical monuments in the gorge attest to the historical importance of the area during the Second Bulgarian Empire (1185–1396). The distinctive appearance of Prohodna Cave—with its eye-shaped openings—likely gave rise to local folklore and religious associations that persist in the Bulgarian 'Eyes of God' name. The gorge served as a traditional route through the Balkan Mountains, and the railway line following the Iskar River was an important engineering achievement of the late nineteenth century, bringing increased access to the region.

Prohodna Cave was designated as a Natural Monument under Bulgarian nature protection legislation in recognition of its unique geological character and scientific importance. The cave and the broader Karlukovo karst area are part of the wider protected landscape of the Iskar Gorge, which has been recognized as an Important Bird Area and is included within Natura 2000 protected areas covering both birds and habitats. The cave has become one of the most photographed natural sites in Bulgaria, with the images of the eye-shaped openings appearing widely in tourism promotion materials. Management of the site has focused on balancing the high volume of tourist visits with the need to protect the cave's ecological and geological values.

Prohodna Cave is the central attraction of the Karlukovo area and is easily accessible on foot from the village of Karlukovo or from designated parking areas near the cave entrance. The walk through the illuminated cave passage is a memorable experience, with the two ceiling openings creating dramatic lighting effects, particularly in the late afternoon when sunlight streams directly through the 'eyes'. The surrounding Iskar Gorge offers extensive hiking possibilities, with marked trails along both banks of the Iskar River and ascending to the plateau above the gorge. Rock climbing on the limestone cliffs of the gorge is popular with experienced climbers. The Karlukovo Monastery, partially built into a cliff face, is a short walk from the cave and provides architectural and historical interest.

Prohodna Cave is located near the village of Karlukovo, approximately 60 kilometers north of Sofia on the road and railway line through the Iskar Gorge. The site is accessible by public transport, with train services stopping at Karlukovo on the Sofia-Vratsa line. Parking is available near the cave entrance for visitors arriving by car. Entry to the cave is free, and no guide or specialized equipment is needed. Basic visitor orientation is provided at the entrance. Several accommodation options are available in and around Karlukovo, including guesthouses and camping facilities along the gorge. Restaurants and small shops operate in the village. The site is easily reached as a day trip from Sofia.

The high visitor numbers at Prohodna Cave create significant management challenges. The cave's bat colonies are sensitive to disturbance, particularly during roosting and hibernation periods, and the presence of large numbers of visitors throughout the warmer months has caused measurable reductions in bat activity. Waste management around the cave entrance and in the broader Iskar Gorge is an ongoing challenge. Physical erosion of cave formations and the cave floor by visitor foot traffic requires monitoring and potentially some visitor flow management. The cave and its surroundings benefit from Natura 2000 protection, and the designation as a Natural Monument provides a legal framework for conservation. Balancing the cave's important role as a tourism asset for the local community with the need to protect its ecological values is the central long-term challenge.