Searching for the secrets of extinction
13 October 2011 Last updated at 10:42 BST
Five times in the last half a billion years, tremendous, global-scale extinctions have wiped out a significant fraction of life on Earth.
The most recent and most familiar is the extinction that wiped out the dinosaurs - between the Cretaceous and Tertiary periods, about 65 million years ago.
But before that, 205 million years ago, was the "End-Triassic Event" - it set the stage for the Jurassic Period, which saw the rise to prominence of the dinosaurs.
Just what happened that killed off half the species on the planet, though, remains a mystery.
One researcher from the US has visited the UK to hunt for evidence of this event - and what he finds may undermine what he had built up for years.
Science reporter Jason Palmer went to Somerset to meet Professor Paul Olsen of Columbia University.