If you were to visit Chicago's Sears Tower, the tallest building in the United States, you would climb 2,232 grueling steps from the ground floor to reach the roof (or save your breath and take a high-speed elevator).You would be far above everything else around you.
Now, from the roof, imagine walking up yet another 40,000 steps--nearly 18 more Sears Towers! That would put you at more than 27,000 feet above sea level, as high as some of the tallest mountain peaks in the ...