Plot synopsis: This astounding documentary delves into the mysteries of the Tunguska event – one of the largest cosmic disasters in the history of civilisation. At 7.15 am, on 30th June 1908, a giant fireball, as bright the sun, exploded in the sky over Tunguska in central Siberia. Its force was equivalent to twenty million tonnes of TNT, and a thousand times greater than that of the atomic bomb dropped on Hiroshima in 1945. An estimated sixty million trees were felled over an area of over two thousand square kilometres - an area over half the size of Rhode Island. If the explosion had occurred over London or Paris, hundreds of thousands of people would have been killed.