The Rookery, an iconic building in Chicago's financial district, was completed in 1888 and designed by the architectural firm of Burnham and Root. Notable for its combination of Moorish, Romanesque, and Venetian architectural styles, the building's most famous feature is its light-filled central atrium, redesigned in 1905 by Frank Lloyd Wright. Beneath this historic landmark lies a little-known underground vault, a remnant from Chicago's early days. Originally intended for utilities and as a foundation to support the structure above, this vault adds a layer of mystery and historical intrigue to the Rookery, symbolizing the city's innovative spirit and complex architectural legacy.