Budapest has long been known for its thermal baths, but below its surface lies an entirely different world. There are two vast, water-filled systems, one carved by nature and the other by miners. These underground spaces, shaped over centuries by shifting rock and human hands, have become unlikely dive sites filled with geological and industrial history.
Their depths offer a view of the city through submerged passageways lined with minerals, machinery, and remnants of lives once lived above.
A Thermal Cave Still Carving Itself Out
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At the base of Rose Hill in the Buda district, Molnár János Cave stretches nearly six kilometers under the surface. It formed as geothermal water dissolved the city’s limestone bedrock over thousands of years. The cave reaches depths of around 90 meters and holds water that remains at 27°C throughout the year.
This cave remains geologically active. Its waters carry hydrogen sulfide and carbon dioxide, which continue to erode the rock and expand the passageways. The interior contains mineral deposits, smooth walls, and areas marked by crystal formations. Fossilized seashells and sea urchins, remnants of the ancient Pannonian Sea, are embedded in some of the rock surfaces.
Controlled Access for Technical Divers
Only certified cave divers can enter Molnár János. Entry begins near a 19th-century building beside a small pond. A corridor leads to the water, where the temperature change is immediate. Inside the cave, divers follow a suspended guideline to prevent disturbing the soft floor, which can turn clear water opaque in seconds.
The cave’s conditions allow year-round diving. Visibility is excellent when undisturbed. Divers commonly see a variety of textures in the rock and occasional aquatic life, which includes translucent shrimp. Some areas still feed water to the nearby Lukács Thermal Baths through underground piping.
An Industrial Legacy Beneath Kőbánya
Credit: Wikimedia Commons/Vince B
On the Pest side of the city, there’s a different structure beneath the Kőbánya district. This is not a natural cave but a 32-kilometer system of cellars created by limestone mining that began in the Middle Ages. The stone extracted here helped build major landmarks, such as Hungary’s Parliament building.
After mining ended in the late 19th century, the chambers remained unused until they began flooding in the 1990s. Experts were brought in to inspect the submerged areas. The site later became accessible for recreational diving under supervision.
Diving Into a Forgotten Underground Facility
One accessible entry point in Kőbánya is Park kút, a flooded chamber within the old limestone mine. Unlike natural caves, these spaces were carved by human hands, and some still contain pockets of fresh air. That makes this site suitable for divers with standard Open Water certification. The water stays around 12 °C, so proper thermal gear is essential even in this relatively beginner-friendly area.
However, the other sites in the mine require advanced certifications and involve enclosed, deeper conditions.
Where the Kőbánya mine is concerned, the deeper you go into it, the more the past starts to show itself. Rusted equipment sits untouched, staircases lead nowhere, and chunks of limestone remain half-cut; left behind when the work suddenly stopped.
During World War II, this underground space became a shelter and even housed a facility for assembling aircraft engines. Hidden in the darkness are three hand-carved churches, one of them surprisingly detailed with Gothic-style arches still visible in the stone.