Transom Mountains
Dimensa
Can be home to characters.
The mountains are tall enough to reach into space, and are suspected to be what gives Dimensa its own section of 'outer space', which some spheres lack. They are impassable from the ground, and difficult to mine through, and whatever sphere might lie beyond them lacks any outer space section to allow access from above.
Thick forests cover the mountains, and snow covers the tallest parts of them that are actually within the atmosphere of Dimensa.