The Dinaric Arc is a vast network of underground caves, lakes and rivers that runs throughout the Balkans.
The labyrinth may be dark and largely devoid of light, but it is full of life - containing the richest variety of subterranean fauna in the world.
This week One Planet heads down into the Arc, to meet some of the creatures that live down there, and also chat to locals about the persistent dumping of waste into the caves, and plans to build a hydroelectric power plant in the underground network.
As ever, tune in, have a listen and then let us know what you think. You can email the team at email@example.com, or join us on Facebook, the link's below.