hollowed out to create the atrium for the expanded building, and new columns were installed behind the existing facade. Large "mega columns" extend downward from the perimeter of the tower addition. The existing frame and new columns are connected with beams at the third and seventh stories. Additionally, there are eight "super-diagonals" sloping from the third floor to the tenth floor. The Hearst Tower contains twenty-one elevators in total. The stairways and elevators are placed in a service core along the western portion of the tower, the only side that does not face a street. The original plan had called for the core to be at the tower's center, but it was redesigned after the September 11 attacks in 2001, as a security precaution against possible attacks from the street. The offset core also enables the office floors to contain an open plan without interior columns. To accommodate the offset service core, and to compensate for the lack of interior columns, the tower's weight is supported by the exterior diagrid, which is braced with the service core. The underlying layer of bedrock beneath the Hearst Tower varies in depth, so the Hearst Tower's foundation was built using two methods. Beneath half of the site, the rock layer is a few feet beneath the basement, and spread footings are used. Under the other half of the site, where the rock layer descends a maximum of beneath the basement, twenty-one caissons were installed. Interior The Hearst Magazine Building initially contained office space with ceilings, which was replaced with a atrium when the tower was built. The atrium has a volume of . The lobby, accessed by escalators from the Eighth Avenue entrance, was placed inside what had been the third story of the original building. The escalators run through a waterfall called Icefall, which uses recycled water from the building's green roof. The waterfall is complemented by a fresco painting, Riverlines, by artist Richard Long. Also within the atrium are two mezzanines; one contains a 380-seat cafeteria and the other houses an exhibition area. The cafeteria, called Cafe 57, is used by Hearst employees and visitors. On the northern side of the atrium is a screening room. At ground level are two storefronts beneath the atrium: an anchor space with about , as well as another space with about . The tower stories start with the tenth story, which is high, slightly