The Mountain Passage Cave was a hole used to be found in the Taiga Mountain Passage, and when entered lead into a very dark cave. This cave should not be confused with the Snow Cave. When entered the cave is very dark, therefore it's recommended to either bring a worklight or adjust your screen brightness somehow.
The cave is fairly small, however, it consists of a tiny little crack in the wall with a cliff, and also a wide narrow flat area above the right-wall of the cave (which is accessible with 2+ people, or a tree). Your car can easily get stuck in this hole or even fall completely down. It should be noted that once you go down in this cave, you cannot get back up without another person or a tree.
This cave was accidentally added in the January Mega Update along with the Taiga Mountain Passage. At the time of its creation, most people thought of it as nothing, as time went on more and more people started to speculate about the reason of its existence along with if it would ever or if it did serve a major purpose in the game. But the cave was most likely just an accidental creation when Defaultio created the Taiga Mountain Passage. On February 15th, 2017, the Taiga Mountain Passage was extended fairly a lot by making most rocks thicker and adding a whole new area at the very top of the passage called the Taiga Peak. Along with the expansion of most rocks the taiga mountain passage cave was blocked off by a rock which had been extended. It is possible that half of the cave still exists completely underground.