In 1912, in a hot air balloon about three miles above the ground, an Austrian scientist called Victor Hess made one of the most astonishing discoveries in science. Up here Hess found that incredibly mysterious rays of energy were pouring in from outer space and streaming through the Earth. They were incredibly powerful, yet unlike anything seen before. They were called cosmic rays. At the same time in laboratories down below, scientists were studying equally mysterious and powerful energy rays pouring out from the interior of atoms - known as radioactivity. Mysterious rays from the vast emptiness of space and mysterious rays from deep within the atom, the tiniest building block. No one really understood what they were or if they might be connected. Then an incredible story unfolded. Cosmic rays and radioactivity turned out to be connected in a way so shocking that it beggars belief. The discovery of this connection would force us to rethink the nature of reality itself.