During World War II Nicholas Alkemade was a tail gunner in a British bomber that was caught on fire and was going to crash. He was forced to jump from the plane without a parachute, falling 18,000 feet. Fir trees and snow broke his fall, and he survived, suffering just a twisted knee.

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Watch Your Step: 6 Things You Can Fall Into

This world is not made for the weak—neither in society nor in the physical world. There you are, making your way across the face of the earth day after day, trusting that, at the very least, the ground under your feet is solid. But is it? As it turns out, you’ve really got to keep your eyes open