A 17,000-year-old tusk contains a remarkable story of the lifetime travels of a woolly mammoth, which roamed the grasslands of Ice Age Alaska. The animal travelled 70,000 kilometres over the course of three decades before his premature death north of the Arctic Circle. The University of Alaska's Matthew Wooller tells Victoria Gill how his team pieced together the mammoth's life from isotopic clues captured in the tusk. Also in the programme: The search for storage capacity underground for all the hydrogen we'll need for a net zero carbon economy, with geoscientists Katriona Edlmann and Eike Thayson of the University of Edinburgh. How the 1987 Montreal Protocol (which phased out CFCs) saved us from an even worse climate crisis than the one we're facing, with climate scientist Paul Young of the University of Lancaster. Probiotics may protect corals from death by bleaching, with marine biologist Raquel Peixoto of King Abdullah University in Saudi Arabia.