Medicine: The Fastest Man on Earth

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The mechanical voice of the loudspeaker cracked across the clear, dry air of New Mexico's Tularosa Valley: "Ten, nine, eight . . ."

Safe in a concrete bunker, tense men at a periscope window kept their eyes on Sonic Wind No. 2, a squat, steel sled with the menacing look of a robot spider. Beneath its red-and-white-striped cab, a string-straight rail track ran across the shimmering heat of Holloman Air Force Base. A patch of blue water dammed up between the rails stretched toward the end of the line, 3,500 ft. away.

"Seven,...