The Mountain biome contains mountains, dense forests often patrolled by robots, and probably a balloon that can quickly reach the top of any mountain. There are fewer houses than in other biomes, but the ones present are mostly big. Usually, many robots roam around.
If you are on top of a mountain and there is a cliff blocking your path and robots or a balloon are nearby -- or you just want to get down -- you can safely descend without fall damage by slowly inching forward so you catch yourself on the side of the cliff (you can't slide down) until you get to the bottom. If done right, you will get down quickly and safely.