In the 19th century brave Mongolian nomads rode icebergs north on Siberia's Yenisei River, trying to reach Tuva, a 65,830-square-mile Shangri-la ringed by mountains. Many died, drowning or freezing to death from temperatures 40 degrees below zero. Nowadays you can just fly there. But famed physicist Richard P. Feynman, the Nobel Prize winner who worked on the Manhattan Project, couldn't. Feynman delighted in solving problems. When the space shuttle Challenger exploded in 1986, he helped solve the riddle of its demise. For fun in his spare time, he cracked safes and picked locks. But he couldn't crack Tuva despite a decade of effort; he died in 1988 without ever being allowed to travel there.
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