Metal at 40

In 1968, Britain's first heavy metal bands were formed, transforming rock music around the world, forever.

Led Zeppelin, Black Sabbath and Deep Purple each had their own unique sound, but shared an approach that embraced "heavy" guitar riffs, powerful lead vocals and thunderous percussion.

Since then, heavy metal has flourished and diversified, spawning sub-genres such as grindcore, thrash and death metal, ever eager to embrace extreme subject matter.

Horror film actor and writer Emily Booth tells the story of the loudest, darkest form of rock, with the help of many of metal's leading musicians.

Producer: Bob Dickinson

First broadcast on BBC Radio 4 in January 2008.