Caves, mines and secret places have long fascinated us. And it's not just the promise of hidden resources that drives us to explore the world beneath the surface. This documentary accompanies rescue workers, underground railway builders and potholers below ground, but we also travel to where nobody has ever been: the center of the Earth. And that's not all: a deadly danger from the depths is lurking undetected inside many German homes. It's time for a voyage of discovery.