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Also called post-thrash, and as the name suggests, it's an extreme metal formed after thrash made its wave.

It blended heavy metal, thrash metal, and hardcore punk together and it came into being in the early 1990s thanks to bands such as Sepultura, Machine Head, White Zombie, Exhorder, Fear Factory, and Pantera.

Just like grunge and (eww...) rap, groove had alot of popularity. There are barely, if not no, groove metal bands that suck ass and any of them are worth a listen even if you are more into grunge or whatnot.

Groove metal was a MAJOR influence to nu metal. Oh yeah about that...
{to nu metalers}
Sepultura, Fear Factory, and Machine Head (ya know, major groove metal bands) experimented with nu metal during it's rise to popularity... however, they did not like their results when that happened and stuck with their old material. Where's your god now, losers?