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Trekking hundreds of miles across a desert is generally an activity reserved for the desperate, but during the Four Deserts Ultramarathon, participants actually choose to run some 150 miles through the world's most inhospitable areas. These extreme athletes carry their own food and equipment and sleep in tents during the race's first three stages, which are held in the Gobi Desert, the Sahara, and the Atacama. Complete two of the first three and your reward is an invitation to take part in the final stage, a walk across Antarctica.