Mushroom Fields

The Mushroom Fields biome, known as the MushroomIsland biome before 1.13, is a very rare lush biome, and is the rarest non-variant biome in the game (the overall rarest biome being Modified Jungle Edge). It usually generates as a single island surrounded by ocean, though occasionally generates touching land, and very rarely generates completely landlocked. It has only one variant.

Description
Mushroom fields biomes consist of a mixture of flat landscape and steep hills and have mycelium instead of grass on the surface. Any grass blocks that are placed by the player in this biome, however, take on a bright green color, similarly to the grass found in jungles. A large amount of brown and red mushrooms cover the mycelium. It is one of the only biomes where huge mushrooms generate naturally, generating as trees, with a density high enough to be a reliable source of huge mushroom blocks, but low enough not to significantly obstruct vision. Here, mushrooms can grow in full sunlight. No mobs other than mooshrooms spawn naturally in this biome, including the usual night-time hostile mobs; this is also the only biome where mooshrooms can spawn. This also applies to caves, abandoned mineshafts, and other dark structures, meaning exploring underground is mostly safe. However, spawners will still spawn mobs, and the player will still be able to breed animals and spawn mobs using eggs; additionally, lightning strikes in this biome still carry the small chance of spawning skeleton trap horses.

This biome is one of the toughest for survival if one does not bring their own resources, especially because of the extreme rarity and non-renewability of wood, with abandoned mineshafts (which also contain poisonous spider spawners) being the only source. Once the first seven wooden planks blocks are gathered however, food is relatively easy to collect, as mooshrooms will produce mushroom stew anytime the player right-clicks them with a bowl. Lack of mobs is a double-edged sword, as while players will never have to worry about being attacked by hostile mobs at night, it also means lack of any mob-related resources unless dungeons happen to generate underground (even if they generate, almost all resources are still unobtainable), some of which are crucial for survival. As an unprepared player cannot make a bed to sleep or bows and arrows to fight, phantoms will likely kill them if they attempt to leave the biome after an extended stay.

Variants
Mushroom fields have only one variant, for a total of two different mushroom fields biomes.

Mushroom Field Shore
Mushroom Field Shore (known as MushroomIslandShore before 1.13) is a technical biome, that represents both the shores and the rivers of the Mushroom Fields biome. It generates when a river cuts through a mushroom fields biome as well as when the mushroom fields biome borders an ocean biome, unless the ocean is a deep variant, in which case a steep cliff will generate instead. The terrain of this biome is much flatter and shallower in elevation than the main mushroom fields biome, though it is equal to the main mushroom fields biome in every other way.