A remote lake in Southeast Asia conceals evidence of Earth’s greatest volcanic cataclysm of the last 100,000 years. Miles beneath its placid surface lies a magma chamber that exploded so violently during the Ice Age that gases and ash may have encircled the globe and blotted out the sun for years on end. The Toba eruption may have helped kick the climate into an unprecedented freeze and perhaps even pushed ancestral human populations to the brink of extinction. In a classic science detective story, NOVA pieces together the clues about this great catastrophe and probes questions raised about human evolution and Earth’s fragile ecosystems.