This is the profile of an extraterrestrial mass murderer: one whose existence was denied by scientific orthodoxy for nearly two decadesbut has now been tracked down. 65 million years ago a 15 kilometers wide asteroid hit the Earth. In 1978 Walter Alvarez, a Nobel prize winning physicist, and his son Luis, first proposed the outrageous idea that a meteorite strike blasted the dinosaurs into extinction, taking with them half of life on the planet. Their theory was hotly disputed. Now the irrefutable evidence is rolling in. Martin Belderson’s dramatic film retraces the hunt for evidence for the hidden “smoking cannon” the crater left by the impact 65 million years ago. In 1991 it was foundburied beneath the coastline of the Yucatan Peninsula, Mexico. The central crater is at least 180 kilometers wide and nearly 20 kilometers deep. The outer impact rings may stretch to 300 kilometers in diameter, making it the largest crater produced in the inner solar system in the last four billion years. But did it cause extinction? The fossil record seemed to show a gradual decline and extinction of animals like the dinosaur before the line of the impact. The in fighting forced many scientists to reexamine their fossil beds. What they discovered has overturned the orthodoxy and led to a new understanding of how extinction works, and why some species survived whilst others vanished. Alvarez, father and son, have finally been vindicated. Horizon reaches back in time and reconstructs how, in a brief moment, the fate of millions of species was sealed: from the new found fossil “horizon of death”to scientists modelling the blast itself, a million times stronger than the world’s combined nuclear arsenals. Startling graphics reveal how in 30 seconds North America was scoured by a fireball. Within an hour the world was aflame. And new evidence shows that the asteroid chanced on the worst possible site on Earth to strike. It hit unique sulphate rich rocks, vaporised them, and kicked billions of tons of sulphuric acid into the atmosphere. The blast fashioned a dense, dust shrouded sulphurous atmosphere which led to darkness for six months. Global temperatures stayed near freezing for a year. The shock heating of the impact created clouds of nitric acid that fell as rain. The top 100 meters of the oceans became stagnant acid, dissolving the shells of sea creatures, including plankton: the base of the food chain. And the culprit? An object 15 kilometers across was needed to do the damage. That meant, not an asteroid, but a comet. There is evidence that 65 million years ago, there was a shower of comets closing on the Earth. What’s more, comet showers are cyclical, returning to threaten the planet at long intervals. Such repeating showers may thus explain many of the other mass extinctions of life on Earth. The hunt is on for more craters.