Since centuries many mysteries have twined around the Blautopf in the Schwäbische Alb, a source which was considered unfathomable for a long time. In September 2006, three speleologists set off for an expedition aiming to unveil one of the last secrets of the blue karst source. They intend to dive through the dangerous underwater cave to penetrate further into the dark and so set foot on places where men have never been before. They do not only discover new caves, but also findings of the late medieval age in the source funnel, and creatures which have managed to survive the Ice Ages in the cave. The cave also offers information about the climate history. The film team accompanied the work of these courageous and curious cave explorers and presents unique shots of a spectacular expedition into the mystical underworld.