Translation:It's very quiet on that mountain.
It should not be wrong if you pluralize it: "in the mountains". If you use singular it really sounds like "the inside of the mountain", which is not what this means.
Another example: 湖の中（みずうみのなか）"in the [middle of] the lake". This is not the very center of the body of water that makes up the lake, but just the middle at surface level.
I believe one would use 真ん中 ("true center/middle") to specify the cases were the literal inside of a mountain or lake is meant.
However, I agree that (until we get a native speaker to chime in) this is technically an interpretation. The lack of articles (and personal pronouns, among other things) in Japanese, combined with the lack of context for these little sentences, makes for a very debatable Duolingo course.