Mountains

The Mountains biome, known as the Extreme Hills biome before 1.13, is a common cold biome. It has three variants and one unused variant.

Description
As the name implies, mountain biomes feature steep terrain, and reach higher elevations than any other biome in the game, save for the shattered variants of Savanna biomes. The surface is made mostly of grass and stone, with some oak and spruce trees growing sporadically on flatter areas. Normal mountains generally peak at roughly Y-level 110, though its variants can go higher. As the climate is cold, rainfall changes to snowfall above approximately Y-level 92, and snow layers form on blocks above that point, along with water freezing into ice. Exposed grass and leaves are a teal-green color. Llamas may spawn on mountains and its variants, and are the only places where llamas can spawn aside from savannas. Additionally, emerald ore exclusively generates in mountain biomes below Y-level 32, and infested stone "veins" generate below sea level.

Due to the terrain generation algorithm, mountain biomes have some unusual features and formations compared to other biomes. Floating blocks and even small floating islands are common, as are overhangs and large waterfalls (occasionally, even lavafalls). This can create some quite impressive views at times. These oddities are magnified in shattered savannas.

Survival in mountain biomes can be challenging to beginners, due to the heights often risking heavy fall damage and higher elevations bringing snow and ice, but can be fairly easy once acclimated to the area, and a flat-enough space is found or made for shelter. Wood can be a bit scarce in certain stony areas, but the Wooded Mountains variant has more plentiful supply, and frequently generates within normal mountains. Llamas can also serve as useful pack animals when tamed. Water must be protected by light or a roof to prevent it from freezing when above the snowfall line, however, and the player must be aware of their surroundings to avoid falling off steep cliffs, especially when fighting hostile mobs. Care should also be taken when mining in mountain biomes, due to infested stone occasionally generating; the player might get swarmed with silverfish if they're not prepared.

Variants
Mountain biomes have three variants in-game, and one deprecated variant, for a total of four different biomes.

Wooded Mountains
The Wooded Mountains variant, known as Extreme Hills+ before 1.13, features significantly higher elevations than the regular mountains, peaking at about Y-level 140. Spruce and oak trees generate much more often on the peaks, forming a small forest or grove. While the increased wood supply can be useful, the taller peaks increases the risk of fatal fall damage, especially if this variant generates on the edge of a mountain range and borders a river or other low-elevation biome. This biome also generates standalone as a thick separation when a desert is bordering a snowy biome.

Gravelly Mountains
The Gravelly Mountains variant, known as Extreme Hills M before 1.13, replaces the top few layers of stone and grass with gravel across its landscape. Its peaks are slightly higher than the regular mountains, though not as high as the wooded mountains. Due to the gravel replacing most of the grass, trees generate far less often, making this variant relatively barren. Players should be careful when surfacing from underground in this biome, as the top layers of gravel can pose a suffocation hazard.

Gravelly Mountains+
Gravelly Mountains+, known as Extreme Hills+ M before 1.13, is a rare variant that is similar to the gravelly mountains, but feature the much taller peaks of the Wooded Mountains. The peaks are still covered in gravel, however, and large valleys often generate among the peaks. Trees are still quite sparse in this biome, making this a difficult biome for survival, due to its barrenness being combined with steep terrain.

Mountain Edge
The Mountain Edge variant, known as Extreme Hills Edge before 1.13, is a currently unused variant. Similarly to the Jungle Edge biome, it was a technical biome intended to provide a smooth transition from other biomes to the mountains. The terrain was gentler and not as steep as the normal mountains, with oak and spruce trees growing. This biome was phased out with the 1.7.2 update and no longer generates naturally, though can still be seen using Customized or Buffet world options.