Your eyes meet what was, for nearly twenty-five years, the tallest building on Earth. Completed in 1973 as the Sears Tower, it rises 1,450 feet on Wacker Drive and held the world height record until 1998. Its engineer, Fazlur Rahman Khan, gave it a revolutionary bundled-tube frame: nine square tubes braced together at the base, dropping away at different heights as the tower climbs, so the silhouette steps inward toward the sky.
That structural idea let it go higher with less steel than anyone thought possible. Sears, then the world's largest retailer, built it to house its offices, though the company left long ago, and in 2009 it was renamed for the insurance broker Willis. Watch how the building's width changes as your eye climbs; those setbacks are not decoration but the bundled tubes ending one by one.