High land prices in populous cities have led to the concept of the "vertical mall," in which space allocated to retail is configured over a number of stories accessible by elevators and/or escalators (usually both) linking the different levels of the mall. The challenge of this type of mall is to overcome the natural tendency of shoppers to move horizontally and encourage shoppers to move upwards and downwards. The concept of a vertical mall was originally conceived in the late 1960s by the Mafco Company, former shopping center development division of Marshall Field & Co. The Water Tower Place skyscraper, Chicago, Illinois, was built in 1975 by Urban Retail Properties. It contains a hotel, luxury condominiums, and office space and sits atop a block-long base containing an eight-level atrium-style retail mall that fronts on the Magnificent Mile.