As one of Europe’s most famous cities, Budapest has become a tourist hotspot known for its history, its architecture and its love of water. While most people soak above the surface in the city’s famed geothermal baths, below the metropolis lies a secret subterranean world ripe for exploration. The Hungarian capital is home to over 80 geothermal springs. As this naturally heated water flowed deep below the Earth’s surface, it carved out hundreds of caves, making Budapest home to the largest collection of thermal caves in the world. Though divers have charted more than four miles of tunnels since the 1950s, much remains to be explored.