The Riesending is Germany’s deepest, longest shaft cave. The rocky corridors wind downwards for almost 25km into the Untersberg Mountain, descending 1,200m through a spectacular subterranean landscape. The film joins an elite team of explorers as they journey into one of the most difficult caves in the world, seeking its end point. , the only people ever to have been granted permission to do so. Specialised light-sensitive, lightweight cameras and lighting help provide a glimpse into the incredible underground world many days away from the earth’s surface. The cliffs inside are higher than the world’s highest church tower. The cavers pass through claustrophobic bottlenecks and vast chasms, through tunnels and past torrential waterfalls. They abseil a towering cliff above a black abyss, in a scene that belongs to Jules Verne or Tolkien, and cross an underground lake in an inflatable dinghy. They get soaked by chilly waterfalls and covered in thick mud. They spend days at three degrees, sleeping on beds of rock, without a glimmer of daylight. These cavers are explorers in the true sense of the word. This underground journey is perhaps the greatest adventure that can be found in todays well-mapped and organized industrial world.