The dinosaurs were some of the most successful animals ever to roam the planet. For a hundred and forty million years, their dominance was unchallenged....except by other dinosaurs. Then, all of a sudden they all disappeared. Something must have killed them off. To find out what had done it, scientists began to investigate a layer of rock formed sixty five million years ago. It's seen in mines and rock outcrops around the world. Below this layer there are lots of dinosaur fossils, above it there are none. It's called the KT boundary.

- This is the KT boundary and because it's such a thin, sharp line we know something dramatic must have happened here, some catastrophe and until recently we have almost no clue what happened whatsoever here, so it remained a total mystery.

- Then in 1979 they discovered a clue in the KT boundary. A high concentration of an element called Iridium. Such quantities are extremely rare on earth and usually come from outer space.

- So as soon as you find ten thousand times more Iridium at the very moment when the dinosaurs disappear, you know somewhere on earth a very big impact must have happened by an asteroid or a comet.

- In fact there was so much Iridium, scientists realised the asteroid must have been a staggering ten kilometres in diameter. If the theory was right, the impact would have created a fireball, equivalent to ten billion Hiroshima bombs. The shock wave alone would have destroyed all life for hundreds of miles around.