Chicago Many other architects have left their impression on the Chicago skyline such as Daniel Burnham, Louis Sullivan, Charles B. Atwood, John Root, and Helmut Jahn. The Merchandise Mart, once first on the list of largest buildings in the world, currently listed as 44th-largest (), had its own zip code until 2008, and stands near the junction of the North and South branches of the Chicago River. Presently, the four tallest buildings in the city are Willis Tower (formerly the Sears Tower, also a building with its own zip code), Trump International Hotel and Tower, the Aon Center (previously the Standard
Metropolitan Tower (Chicago) of this ornament, the building is sometimes referred to as the "Beehive Building." Just beneath the beehive are four carillon bells ranging in weight from 1,500 to 7,000 pounds, unused for many years until restored in 1979 for the Chicago visit of Pope John Paul II. At one time, the bells chimed the well-known Cambridge Quarters on the quarter-hours. The base has been altered from its original design: rectangular window openings replaced giant arches on Michigan Avenue and Jackson Boulevard. At one time, the thirtieth floor was the Straus Tower Observatory, which was open to the public for viewing the