Original Metallica guitarist Lloyd Grant spoke in an interview with The Ex Man Podcast and revealed the unheard story about how they created “thrash metal”. He said (transcriped by Ultimate-Guitar):

“Well, ’81-’82 is just about when we started playing. And one day I went over to Lars’ apartment – his parents lived in Newport Beach. We go over there and he says, ‘Hey, some guy [James Hetfield] sent me a tape.’

He played it for me and it was ‘Hit the Lights’ [which James originally wrote with his old band Leather Charm]. It was right in our alley – that’s what we would like to do, and that started it.

You listen to the compilation album [Metal Blade’s 1982’s compilation “Metal Massacre I,” the first release to ever feature a Metallica song, which was “Hit the Lights”], there wasn’t any band that was doing stuff like that.

It was a mixture between punk and heavy metal. And that became speed metal. But it was not like LA glam rock or hardcore punk, it was right in between.”

Click here to entire interview.