Metallica's debut album and the one that officially launched the thrash genre, arguably what would become the biggest branch of metal, came out this day in 1983. It wasn't an album that exploded and lit up the charts but it was a big deal in underground circles at the time and spread pretty quickly considering there was no internet at the time. While other bands like Exodus, Anthrax and Slayer were also already playing in their local areas Metallica was the first to record an album and get it out the door. In fact Kerry King of Slayer directly credits an early Metallica gig he saw with Dave Lombardo as influencing Slayer and the direction they wanted to go.
(An incredibly early pic back when Dave Mustaine was still there)
There's really not much else that needs to be said most people know what happened after this. They continued to get bigger and bigger with each album and their success led to many other thrash bands getting record deals as competing labels wanted "their own metallica". It's not my favorite album of theirs but it was and still is an incredibly important album in music history. There's not many albums that can take credit for launching a whole genre and inspiring dozens of contemporary and future bands, many who are legendary in their own right. If you want to find out more about that early period in thrash I recommend checking out the murder in the front row documentary it covers most of what went on in san francisco in the early to mid 80s.