Molnár János Cave

A largely water-filled, actively forming thermal cave beneath the Buda side of Budapest, surveyed to about 6.2 km by the end of 2025. Recreational diving is offered only as guided dives through one licensed dive centre.

The Molnár János Cave opens on the Buda side of central Budapest, where warm spring water feeds the artificial Malom-tó (Mill Lake) beside Frankel Leó út. Almost the whole known cave lies below the water table, so nearly all exploration and survey work has been done by divers.

The cave is named after János Molnár, a 19th-century pharmacist who explored the small dry section above the lake and studied the spring water. Diving exploration began in the 1950s; in the 1970s and 1980s the FTSK Delfin club explored and mapped more than 400 m of passage. A breakthrough in 2002 opened large new sections, including a hall about 86 m long, 27 m wide and 15 m high, and the surveyed length has since grown to more than 6 km.

For visiting divers the cave is unusual in two ways: it sits inside a capital city, and the water is warm by cave-diving standards, with measured temperatures between roughly 20 and 26 °C in different passages. Diving is organised exclusively as guided dives by the licensed on-site dive centre, which publishes its own equipment rules, group-size limits and prices.

The cave is strictly protected. Scientific work (water chemistry, flow measurements, biology and new exploration) continues under research permits issued by the Duna-Ipoly National Park Directorate, and the research team files annual reports with the national nature conservation authority.

The cave is described by its operator as a hypogenic cave: it was dissolved from below by rising thermal water, mainly through mixing corrosion where water bodies of different temperature and chemistry meet along fractures. Passages follow tectonic lines in Upper Eocene limestone and marl (the Buda Marl), and the cave is still actively forming.

Spring water discharges from the cave into Malom-tó, which drains through a sluice and canal towards the Danube beneath the Lukács Baths. Temperatures inside vary by passage: in autumn 2025 the research team measured about 20 to 26 °C, with the warmest water in the Kessler Hall and cooler inflows elsewhere. The same report describes a newly observed temperature layering near the old entrance from August 2025, with a cooler bottom layer flowing back into the cave.

Listed by showcaves.com as the longest active thermal cave in Hungary, and a long-running research site for thermal-karst hydrology and water chemistry (sampling series since 2014). Three Hungarian Niphargus species (small cave crustaceans) and other aquatic invertebrates have been recorded.

Thermal water, warm for a cave. The operator gives 20-28 °C year-round; 2025 research measurements ranged from about 20 to 26 °C depending on the passage. The operator reports very slow flow and no strong currents. Its rules allow wetsuits only in Zone 1; deeper zones require a drysuit.

Typical ranges from published sources, not current conditions.

The diving entrance is the spring in Malom-tó at Frankel Leó út 48, used by the licensed dive centre. A second, dry entrance on the hillside leads down to the water table. Divers enter only via the dive centre.