- [Host] 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.
(mysterious music)
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 by János 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.