What is the difference between "on the mountains" and "in the mountains"?
“in the mountains” means to be in the vicinity of mountains or in a mountainous area. “on the mountain” means to be literally on the surface of a specific mountain. (I also thought that the latter phrase was mountaineering jargon, but I can’t find evidence of this when googling.) I don’t think you’ll find “on the mountains” too often, outside of something like, “We left our gear as we went, so in a sense, parts of us were left on the mountains.”