In a world where we can examine just about every part of the planet's surface with Google Maps, it sometimes feels ... Rick Steves' favorite cave system is a hidden gem tucked away in Slovenia.

from which surface water makes its way underground, eroding the limestone into a honeycomb of caverns. Mammoth, the world's longest known cave system, a UNESCO World Heritage site and the core ...

From intricate cave formations to the deep sense of history hidden within their walls, these underground wonders ... this cave system has been enjoyed by thousands since it opened to the public ...