 You're swimming through a narrow corridor in an underwater cave, and your air is almost out. With nowhere to turn but yourself. You're flying 40,000 feet above the Amazon rainforest, looking down at the jungle below. And you need to make a choice. An avalanche of snow is heading straight down. Water starts pouring into the ship. The plane dips down below the trees, hurtling towards the ground. Survive? 20,000 feet. 10,000. Five. Or die. Mayday.