5. While the Sears Tower is arguably the greatest achievement in skyscraper engineering so far, it's unlikely that architects and engineers have abandoned the quest for the world's tallest building. The question is: Just how high can a building go? Structural engineer William LeMessurier has designed a skyscraper nearly one-half mile high, twice as tall as the Sears Tower. And architect Robert Sobel claims that existing technology could produce a 500-story building. From Ron Bachman, "Reaching for the Sky." Dial