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  • Budapest, Hungary, one of Europe's hidden jewels.

  • A tourist hotspot known for its history and beautiful architecture and its love of water.

  • But while most splash above the surface, below the city is a secret subterranean world.

  • [Caves under the capital]

  • Hungary's capital is home to over 80 geothermal springs and dozens of baths, all heated by the Earth's warmth deep below the surface.

  • As this naturally heated water flowed beneath the city, it carved out hundreds of caves, making the city home to the largest collection of thermal caves in the world.

  • The crown jewel of this system is the Molnár János cave.

  • Discovered in the 19th century bynos Molnár, who traced the warm spring waters from the lake outside to its source beneath the city.

  • Divers first started to explore the cave in the 1950s.

  • Since then, nearly four miles of tunnels have been charted.

  • (gentle music)

  • In the pitch black of the cave, miles of rope have been laid for divers to follow the correct route back to the exit.

  • The water is naturally warm, between 70 and 80 degrees fahrenheit.

  • The rooms of the cave can range from tiny to massive.

  • The largest is so large, it can hold 350 double decker buses.

  • And even in the pitch black, life has found a way.

  • Biologists have identified new species that live only in these caves.

  • Much is still to be answered about this place, a reminder that even in the most urban of areas, you can still find some of nature's greatest wonders.

Budapest, Hungary, one of Europe's hidden jewels.

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