People are coming down a movable staircase built through a tunnel, which has three of those staircases.
People are climbing a mountain.
People using a movable staircase in sentence 1 and a mountain in sentence 2.
Climbing a mountain implies that there is no staircase, and the tunnel implies that it is manmade where as the mountain implies its natural.
If they are coming down a movable staircase, they cannot be climbing a mountain simultaneously.