About 250 million years ago, something killed off 95% of life on Earth, the worst extinction the planet has ever recorded. This episode of the Catastrophe series, hosted by Tony Robinson, follows scientists testing the old asteroid-impact theory against a newer, slower culprit: the Siberian Traps, a vast eruption of flood basalt that poured out over tens of thousands of years. Filming in South Africa, California, and Iceland, researchers read the story out of the rock itself, tracing how volcanic gases acidified rain, choked oceans with hydrogen sulfide, and pushed global temperatures up until deserts replaced forests. Fossils of Dicynodonts and Gorgonopsians mark what the world looked like just before the collapse. The food chain breaks down link by link on screen, with even large, hardy animals unable to survive the shift. Only 5% of species made it through, and their descendants eventually produced everything alive today. The episode closes by asking whether current carbon dioxide emissions are quietly rebuilding the same conditions, with one scientist answering that the only open question is when.