"in the mountain" vs "in the mountains"

[shikemoku]

Could someone tell me whether the below sentences are correct, and if both are correct, what the difference is?

"I enjoyed walking in the mountain."
"I enjoyed walking in the mountains."

I know they usually use "in the mountains."
But if you talk about a walk in a specific mountain, is it OK to use the singular form?

Thank you in advance.

 

[GoesStation]

"I enjoyed walking in the mountain."
"I enjoyed walking in the mountains."

I know they usually use "in the mountains."
But if you talk about a walk [STRIKE]in[/STRIKE] on a specific mountain, is it OK to use the singular form?

It's never okay to use the singular form unless you have been in tunnels within a mountain. This is the unlikely situation Robert alluded to in post #2.

 

[shikemoku]

Thank you, RobertJ and GoesStation.

So we can't say "in the mountain" unless it's literally "in" the mountain. Is it right?
Is it possible to say, "I enjoyed walking on the mountain.", if we talk about a specific mountain, for example, Mt. Fuji?