Thanks for down learning the In Our Time podcast. For more details about In Our Time and for our terms of use, please go to BBC.co.uk.

I hope you enjoy the program.

Hello, across the entire planet where it hasn't been eroded or destroyed in land movements,

there's a thin grey line of clay.

In Italy it's 1 centimetre thick, in Central America it stretches to 3

centimeters but it's all the same thin gray line laid into the rock some

65 million years ago and very likely it bears witness to a

cataclysmic event experienced very rarely in Earth's history.

It's called the K T Boundary, and geologists believe it's the clue to death of the dinosaurs

and the key reason why mammals and humans inherited the Earth.

But exactly what did happen 65 million years ago.

How is this extraordinary timeline the K.T. Boundary created across the Earth?

With me to discuss the boundary is Simon Kelly,

head of Department of Earth Sciences at the Open University.

Jane Francis, professor of paleoclimatology at the University of Leeds,

and Mike Benton, professor of Earth Sciences at the University of Bristol.

Simon Kelly, can you just tell us what the KT boundary is?

It's an instantaneous boundary formed at the same time across the entire world. Now that sounds like

something that you would expect, but it's almost unique in geological history. There

isn't another layer of rock that we can find almost everywhere in the world. We find it in America, Denmark,

New Zealand, everywhere. And it consists of clay, broken down glass which breaks clay which breaks down to clay

the layers in America about 2 centimeters thick but inside the layer you can actually in America break it down.

...