Our pilot, David Kunkel, asked me to retrieve his oxygen bottle from under my seat, and when I handed it to him he gripped the plastic breathing tube with his teeth and opened the valve. We had taken off from Boulder that morning, and were flying over Rocky Mountain National Park, about thirty miles to the northwest. We were in a Maule M-7, a single-engine “backcountry” plane, and Kunkel was navigating with the help of an iPad Mini, which was resting on his legs. “People don’t usually think altitude is affecting them,” he said. “But if you ask them to count backward from a hundred by sevens they have trouble.” What struck me at that moment was not how high we were but how low: a little earlier, we had flown within what seemed like hailing distance of the sheer east face of Longs Peak, and now, as Kunkel banked steeply to the right to give a better view of a stream at the bottom of a narrow valley, his wingtip appeared to pass just feet from the jagged declivity beneath. Snow had fallen in the mountains during the night, and I half expected it to swirl up in the plane’s wake.