Extinction is a natural process, and five times in the history of life it has happened on a huge scale. The dinosaurs were wiped out by a massive meteor strike that also killed off much of life on Earth. Human activities are beginning to cause a present day mass-extinction that is happening at unprecedented speed. How can we prevent this from happening? David Attenborough investigates which human activities are the most damaging to the natural world, in an attempt to understand how we can prevent this crisis from unfolding.