Szemlő-hegyi Cave was discovered in 1930 during quarry work on Rózsadomb, revealing a labyrinth shaped not by rivers but by ascending thermal water. The cave's pale calcite formations — cauliflower-like clusters called "popcorn" and delicate aragonite needles — formed over two million years as mineral-rich springs rose through fractures in the Triassic limestone. Unlike Hungary's river-carved caves, Szemlő-hegyi belongs to the hydrothermal category, its walls smooth where hot water dissolved the rock rather than abrading it. The cave system remained dry after the thermal activity ceased, preserving fragile structures that would disintegrate in a humid environment. Pál-völgyi Cave, mapped to 31 kilometres, is the longest cave system in Hungary and the second-longest thermal maze cave on Earth. Its passages stack in multiple levels, connected by narrow chimneys and vertical shafts that require metal ladders during guided tours. Exploration continues — surveyors added 1.2 kilometres of new passages in 2019 when a breakthrough connected Pál-völgyi to the neighbouring Mátyás-hegyi system. The cave's temperature holds constant at 11°C year-round, and sections of the tour route pass chambers where nineteenth-century wine cellars were carved into the limestone before the deeper galleries were discovered. Both caves became protected nature reserves in 1982. Szemlő-hegyi gained medical recognition in the 1980s when studies confirmed that its microclimate — stable humidity, low dust, and elevated radon levels — benefits respiratory patients; a dedicated speleotherapy chamber now operates within the cave under clinical supervision. Budapest's underground cave tour routes are maintained by the Duna-Ipoly National Park Directorate, which limits daily visitor numbers to prevent carbon dioxide buildup from breath and body heat. The 400-step descent into Pál-völgyi includes a section called the Radium Hall, named not for radioactivity but for the luminous calcite that early explorers mistook for radium paint. Visitors to the Buda Hills cave system walk through geology that records two distinct epochs: the deposition of marine limestone 220 million years ago when the region lay beneath the Tethys Ocean, and the hydrothermal phase two million years ago when the rising Buda Hills channeled geothermal water upward through fault lines. The caves lie within a 10-kilometre belt of similar formations, though only Szemlő-hegyi and Pál-völgyi are open for public budapest underground cave tour access. The combined ticket offers entry to both sites on the same day, allowing comparison between the therapeutic stillness of Szemlő-hegyi and the vertical athleticism required in Pál-völgyi's deeper passages.