Extreme metal and underground metal are often used synonym, but technically metal can be underground without being extreme. For example, while power metal has mainstream popularity in europe, it's considered to be underground in the US, and many other metal subgenres are much more underground than power metal, even some extreme metal subgenres, like neoclassical metal. This brings to the next point. Even though undergroundness is supposed to be inherent to the idea of extreme metal, a few extreme metal bands have broken into the mainstream. Aside from the big four of thrash, there are a few individual extreme metal bands like Lamb of God and Cannibal Corpse who are pretty popular for metal bands, and the most popular genre of metal right now among modern bands, metalcore, is based in extreme metal, the popular bands just have softer metal and pop elements to make it accessible. However, for the most part mainstream metal bands are in the non-extreme genres/extreme metal is underground, and the popular bands in the non-extreme genres are more mainstream than the popular bands in the extreme genres, so that brings the question: Can extreme and underground be considered to be the same thing?