You're about to listen to a BBC podcast and I'd like to tell you a bit about the

podcast I work on. I'm Dan Clark and I commissioned factual podcasts at the BBC.

It's a massive area but I'd sum it up as stories to help us make sense of the forces shaping the world.

What podcasting does is give us the space and the time to take brilliant BBC journalism

and tell amazing compelling stories that really get behind the headlines.

And what I get really excited about is when we find a way of drawing you into a subject

you might not even have thought you were interested in.

Whether it's investigations, science, tech, politics, culture, true crime, the environment,

you can always discover more with a podcast on BBC Sounds.

Thank you for downloading the Life Scientific from BBC Radio 4.

My guest today has spent 40 years trying to unravel a mystery at the frontiers of physics.

Cosmic rays are subatomic particles with the highest known energies in the entire universe,

bombarding the earth in a shower of cosmic rain.

As many as a million of them pass through our bodies every night as we sleep.

Some even have energies millions of times greater than anything that could ever be produced at the Large Hadron Collider.

But where do they come from? And which violent

astronomical events are producing such hugely energetic streams of particles that travel for

light years to reach us.

Well, my guest is Professor Alan Watson, who says it's a great embarrassment that the

mystery is still unsolved and we don't know for sure where cosmic rays come from.

His quest to find their origins has taken him from the North Yorkshire Moors

to the South Pole and the Pampas Grasslands of Argentina,

...