Szemlő-hegyi Cave holds the highest concentration of calcite rafts in Central Europe — delicate mineral plates that float on underground pools before calcifying into the ceiling. The cave, discovered in 1930 during quarry work on the western slope of Szemlő Hill, stretches 2,200 metres through Triassic limestone laid down 240 million years ago when the Pannonian Basin lay beneath a shallow sea. Thermal water rising from tectonic fractures carved the passages over two million years, depositing the popcorn-textured aragonite that now coats the walls in a crust resembling frozen cauliflower. Pál-völgyi Cave, three kilometres north, extends 31 kilometres into the Buda Hills — the longest cave system in Hungary and the second-longest thermal cave on Earth. Cavers mapped the first passages in 1904, but the full extent remained unknown until a 1980s expedition connected it to the neighbouring Mátyás Hill system. The cave's upper galleries remain above the water table, preserving fossils of Miocene-era molluscs and the bone fragments of Pleistocene mammals that fell through vertical shafts during the last ice age. The Radium Hall, named for the radon-rich air once prescribed as respiratory therapy, sits 60 metres below street level and maintains a constant 11°C year-round. Both caves served as air-raid shelters during the Second World War and as wine cellars in the 19th century, before speleologists recognised their scientific value. The Hungarian Cave Rescue Service now monitors the passages, and conservation lighting installed in 2014 prevents algae growth while illuminating the scalloped rock where ancient currents sculpted the limestone. Guided budapest castle cave tour experiences run hourly through both systems, with routes designed to showcase the contrasting formations — Szemlő-hegyi's mineral blooms and Pál-völgyi's vertical chimneys. The Buda Castle district above the caves sits on the same karstic plateau, its cellars and tunnels forming a shallow counterpart to the deeper thermal network. Speleotherapy clinics operated in Szemlő-hegyi until 2008, treating asthma patients in the saturated air of the lower chambers. Today the caves remain among the few thermal systems accessible without caving equipment, their reinforced passages fitted with handrails and non-slip grating to accommodate the 400-step descent into the hills.