Mushroom Fields is most peaceful biome in entirety of Overworld (Surface). Main component of this biome is mycelium, a type of dirt which can be found only in this biome. There are only two creatures living here: Brown and Red Mushroom Cows. While this biome is peaceful, it is always after, often near Lava Castle (Which means that you most likely will go through Lava Castle first, a very dangerous Overworld structure).