Fantastic diving beneath Budapest's streets, just a few kilometers from the ocean

Fantastic diving beneath Budapest's streets, just a few kilometers from the ocean

Every day, people flock to the Lukács Thermal Baths in Budapest to relax in warm, mineral-rich pools as the yellow trams rumble along Frankel Leó Street. Most people have no idea that just a few meters away, beneath the city's historic streets, lies a hidden world: a vast submerged cave system heated by geothermal springs.

The Molnár János cave system

The entrance to Molnár János Cave is located at the base of Rózsadomb – Rose Hill – an affluent district lined with elegant villas and streets. The cave stretches for more than 5.8 kilometers and plunges almost 90 meters below the surface. Filled with crystal clear water the temperature of a warm bath, it is one of the largest active thermal water caves in the world, accessible to certified cave divers.

A hidden gem

The entrance to the cave is easy to miss from the road. Beside a limestone cliff, a small lake covered in water lilies and seaweed glistens next to a crumbling 19th-century building that resembles an Ottoman bathhouse. On the wall near the entrance, a mural of a diver tells of the secrets hidden beneath the surface.

Through a gate, past an old bathhouse - once one of Hungary's first concrete buildings - a narrow alley leads to an unmarked door in the cliff. The “Factory of Happiness” sign is emblazoned above the door, flanked by smiling emojis. When you enter the brick-lined anteroom, the temperature rises due to the geothermal heat. Diving equipment is lined up along the corridor. At the end, behind a drawn curtain, a staircase leads into a rocky entrance where the black waters wait in the darkness. Here, divers descend into the warm water, their headlamps piercing the darkness as they plunge into a silent, shimmering world.

A geological wonder

Budapest is famous for its lavish bathhouses and spas, but the thermal springs did more than just relax muscles and soothe aches and pains. Over millennia, the same geothermal activity that feeds the city's baths has carved a network of more than 200 caves beneath its streets, while mineral-rich springs have slowly dissolved the surrounding limestone and marlstone.

Molnár János Cave continues to be active and growing. Water rich in hydrogen sulfide and carbon dioxide continues to seep through the stone, creating a slightly acidic cocktail that eats away at the walls. The result is a labyrinthine network of chambers and passages.

A unique diving experience

“Warm water caves are very rare,” explains Csaba Gőcze, a dive guide with MJ Cave, the local guided cave dive provider. "Normally, cave diving means water temperatures between 4 and 15 degrees Celsius. Here we have 27 °C in the upper layers."

The heat is distributed in different layers: 27 °C at the surface, falling to 20 °C and then further to 17-18 °C, while colder water from the Buda Hills is mixed below. Some of the water in the cave also feeds the nearby Lukács Baths through an underground pipe - although the original inflow was diverted at the stone staircase leading into the water to allow easier access for divers.

A fascinating diving location

Molnár János Cave surprises many first-time visitors. Unlike the narrow, winding passages of other caves, it offers spacious chambers and gentle currents.

“It’s absolutely breathtaking,” says Csaba. "Huge, open spaces and very few restrictions. It's a relatively easy dive - provided you're properly trained."

Training is essential. Only certified cave divers are allowed to dive. The complete darkness and sensitive environment require experience. The water is perfectly clear—until someone touches the cave wall or stirs up the soft ground, sending fine mud particles flying into the beam of light from a headlamp.

Natural wonders and scientific exploration

“You can usually see as far as the light from your lamp allows,” says Csaba. “But if someone touches the walls or stirs up the mud at the bottom of the corridors, visibility drops to zero very quickly.”

To prevent this, divers follow a line stretched three feet above the cave floor to ensure the water remains pristine. Your caution will be rewarded with a tour of a surreal landscape: mineral-rich walls studded with crystals and chambers that change in color and texture from one to the next.

“Everything looks very different in different places in the cave, as you see different color of stones, areas with crystals and those without,” Csaba adds. “The best dives are the ones where you dive through several of these areas to experience everything.”

Crabs – barely visible – scurry around in the light. Fossilized shells and sea urchins still cling to the walls, remnants of the Pannonian Sea that covered much of what is now Hungary millions of years ago.

The discovery continues

The cave has not yet been fully explored. The officially mapped network measures 5.8 kilometers, but new passages are discovered regularly. Volunteer explorers make weekly expeditions into the caves to take measurements, lay new lines and update maps that have not yet been published.

“The official map says it is about 5,800 meters, but there are passages without lines,” explains Csaba. “Some go nowhere, but others could be part of something bigger.” He believes the cave could extend up to eight kilometers.

Scientists are also collecting water samples to monitor microplastics and signs of pollution. A 2022 study discovered some contaminants at known dripwater entry points, but recently explored areas remain untouched and untouched.

It is remarkably easy for certified cave divers to take a dive here. MJ Cave offers morning dives by reservation. After briefing and equipment, the first one-hour dive focuses on the main line, followed by optional deeper explorations - some dives can reach nearly 60 meters and require decompression stops. Traveling to Hungary without equipment is not a problem; Full equipment rentals are available on site.