Vertical.

In the mountains, they stand alone against whatever will happen. For the several days they live in nature, they can rely only on themselves and each other. Elsewhere, there is always a fallback - here one is without a safety. This in itself can bring some to terror - the terror of having to rely on yourself - your own judgment, wits and fortitude. This is also the most incredible feeling of strength and independence. Knowing there is no safety means having to be one's own safety.