A Hidden World Beneath Budapest
Every day, people gather at the Lukács Thermal Baths in Budapest, soaking in warm mineral-rich pools while yellow trams pass by on Frankel Leó Street. Few are aware that just a short distance away, beneath the city’s historic streets, lies an extraordinary underground world: a vast cave system heated by geothermal springs.
Tucked into the base of Rózsadomb, a neighborhood known for its elegant villas and tree-lined streets, the Molnár János Cave stretches over 3.6 miles (5.8 kilometers) and plunges nearly 300 feet (90 meters) below the surface. The cave is filled with crystal-clear water at a temperature similar to a warm bath, making it one of the largest active thermal water caves in the world—and among the few open to certified cave divers.
The entrance to the cave is easy to overlook from the street. Next to a rocky limestone cliff, a small lake covered with lilies and algae glimmers beside a crumbling 19th-century building that resembles an Ottoman bathhouse. A mural of a diver on the firewall hints at the secrets hidden below.
Through a gate and past an old bathhouse—once one of Hungary’s first concrete structures—a narrow alley leads to an unmarked doorway in the cliff. Above it, a sign reads “Happiness Factory,” flanked by smiling emojis. Once inside the brick-lined entranceway, the temperature rises with the geothermal heat. Diving gear lines the corridor. At the end, behind a drawn curtain, a stairwell drops into a rocky entrance where black waters wait in the gloom. Here, divers step into the warm water, their headlamps piercing through the darkness as they descend into a silent, shimmering world.
A Millennium-Old Formation
Budapest is well-known for its ornate bathhouses and spas, but the city’s thermal waters have done more than just soothe muscles and ease ailments. Over millennia, the same geothermal activity that feeds the city’s baths has carved a network of more than 200 caves beneath its streets. As mineral-rich springs slowly dissolved the surrounding limestone, marlstone, and karst rock, these caves formed.
Molnár János is still growing. Water rich in hydrogen sulfide and carbon dioxide continues to percolate through the rock, creating a mildly acidic mixture that eats away at the walls. The result is a labyrinth of chambers and passages, resembling Swiss cheese.
“It’s very rare to have warm-water caves,” explains Csaba Gőcze, a dive guide with MJ Cave, the local operator offering guided cave dives. “Usually, cave diving means 4 to 15 degrees Celsius (39 to 59 Fahrenheit) water. Here, it’s 27 °C (80 °F) in the upper layers.”
The warmth comes in distinct bands: 27 °C at the surface, dropping to 20 °C (68 °F) and then to 17–18 °C (62–64 °F) as colder water from the Buda Hills mingles below. Some of the cave’s water still feeds the nearby Lukács Baths via an underground pipe—though the original inlet was rerouted to give divers easier access.
A Surreal Underwater Landscape
Molnár János surprises many first-time visitors. Unlike the tight, twisting passages of other caves, it offers spacious chambers and gentle currents. “It’s absolutely gorgeous,” says Csaba. “Huge, open spaces and very few restrictions. It’s a relatively easy dive—if you’re properly trained.”
That training is essential. Only certified cave divers are allowed in. The complete darkness and fragile environment demand experience. The water is perfectly clear—until someone brushes the side of the cave or disturbs the soft bottom, sending fine silt particles billowing into the beam of a headlamp. “You can usually see as far as your torchlight reaches,” Csaba says. “But if people touch the walls or kick up the silt on the bottom of the passages, visibility drops to zero very fast.”
To prevent this, divers follow a guideline strung a meter above the cave floor, ensuring the waters remain pristine. Their caution is rewarded with a tour of a surreal landscape: mineral-streaked walls studded with crystals, chambers shifting in color and texture from one to the next. “Several places in the cave look completely different, as you have different colored rocks, areas with crystals, and areas without,” Csaba says. “The best dives are where you go through several of these areas, so you experience it all.”
Shrimp—nearly invisible—dart in the light. Fossilized seashells and sea urchins still cling to the walls, remnants of the Pannonian Sea, which millions of years ago covered much of modern-day Hungary.
Exploration and Discovery
The cave still hasn’t been fully explored. The officially mapped network runs 3.6 miles, but new passages are found regularly. Volunteer explorers make weekly expeditions into the caves to make measurements, lay new lines, and update maps that have yet to be published. “The official map says it’s around 5,800 meters, but there are passages without lines,” Csaba explains. “Some don’t lead anywhere, but others might be part of something bigger.” He believes the cave could stretch to five miles.
Researchers also collect water samples, monitoring for microplastics and signs of pollution. A 2022 study detected some contamination at the cave’s known dripwater entry points, but the recently explored areas remain untouched and pristine.
For certified cave divers, joining a dive here is remarkably straightforward. MJ Cave runs morning dives by reservation. After a briefing and gear setup, the first one-hour dive follows the main guideline, with optional deeper exploration afterward—some dives can reach nearly 200 feet (60 meters) and require decompression stops. Traveling to Hungary without equipment isn’t a problem; full gear rental is available on-site.
