When we think of scuba diving, we usually think of sea level. But for many divers in the American West, the most stunning underwater vistas aren’t at the coast—they are tucked away in the peaks of our mountain ranges. High altitude diving offers a surreal, crystal-clear experience, but it also comes with a unique set of physical rules. Before you head to the mountains, you need to understand how the lower atmospheric pressure changes your dive profile and your safety.