There are three different terrains described in the story. How are these different from each other?

Snafell is a 5,000-foot high volcano. It is inactive, has two peaks, a double cone, and the crater has three chimney-like structures,

Beneath Snafell and Iceland there are tunnels and caverns, which are designed to lead from one place to another. The cavern leads to the underground world.... the subterranean landscape, and a place of light.