Mountains (formerly called extreme hills) is a Biome where abrupt and drastic changes in elevation are common across the landscape.
The mountains biome resembles real-life Highlands, with hills, valleys, mountains, and plateaus. One major difference is that large, overhanging patches of land are commonly generated throughout the biome. This biome contains some of the highest elevations in the Overworld. In some places, the elevation can be so high that Snow will cover the tops of hills. Hill sizes and shapes vary, from gradual-sloping hills to steep vertical cliffs, to bare hills composed of Stone and Gravel. Coal spawns here more common than in other biomes, underground.