Miles from the ocean, there's incredible diving beneath the streets of Budapest

CNN

2 days ago

Every day, crowds flock to the Lukács Thermal Baths in Budapest, soaking in warm mineral-rich pools as yellow trams clatter along Frankel Leó Street. Most never suspect that just yards away, beneath the city's historic streets, lies a hidden world: a vast underwater cave system heated by geothermal springs.
From its entrance, tucked into the base of Rózsadomb — Rose Hill — an affluent neighborhood of elegant villas and tree-lined streets, the Molnár János Cave stretches for over 3.6 miles (5.8 kilometers) and plunges nearly 300 feet (90 meters) below the surface. Flooded with crystal-clear water at the temperature of a warm bath, it is one of the largest active thermal water caves in the world, and among the rare few open to certified cave divers.
The way into the cave is easy to miss from the street. Next to a rocky limestone cliff, a small lake covered with lilies and algae glistens beside a crumbling 19th-century building that resembles an Ottoman bathhouse. On the firewall next to it, a mural of a diver hints at the secrets below.
Through a gate, 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.
Budapest is famous for its ornate bathhouses and spas, but its thermal waters have done more than 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.
Molnár János is still alive and growing. Water rich in hydrogen sulfide and carbon dioxide continues to percolate through the rock, creating a mildly acidic cocktail that eats at the walls. The result is a Swiss cheese labyrinth of chambers and passages.
'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, by the stone steps that plunge into the water, was rerouted to give divers easier access.
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,' Csaba says. '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 the 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.
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 make updates to 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.