The passages of Europe’s largest hydrothermal cave, the Molnár János Cave in Budapest, are still being explored. Its known passages are over 6 km long, the deepest part is 90-100 m, but it is likely that the cave is even longer and deeper. The formation of the cave is due to the so-called mixing corrosion that occurred during the mixing of the water flow systems in the Eocene Szépvölgy Limestone and the Buda Marl. The formation of the cave’s passages has been significantly influenced by tectonic trends. In 2008, an underground lake was discovered in the cave. This body of water stretches for an impressive 86 metres long, 27 metres wide and 15 metres deep.
See the following YouTube video about the mission: