Centered on Post Oak Boulevard and Westheimer Road, the Uptown District boomed during the 1970s and early 1980s when a collection of mid-rise office buildings, hotels, and retail developments appeared along Interstate 610 west. Uptown became one of the most prominent instances of an edge city. The tallest building in Uptown is the 64-floor, 901-foot (275 m)-tall, Philip Johnson and John Burgee designed landmark Williams Tower (known as the Transco Tower until 1999). At the time of construction, it was believed to be the world's tallest skyscraper outside of a central business district. The new 20-story Skanska building and BBVA Compass Plaza are the newest office buildings built in Uptown after 30 years. The Uptown District is also home to buildings designed by noted architects I. M. Pei, César Pelli, and Philip Johnson. In the late 1990s and early 2000s decade, there was a mini-boom of mid-rise and high-rise residential tower construction, with several over 30 stories tall. Since 2000 more than 30 high-rise buildings have gone up in Houston; all told, 72 high-rises tower over the city, which adds up to about 8,300 units. In 2002, Uptown had more than 23 million square feet (2,100,000 m²) of office space with 16 million square feet (1,500,000 m²) of Class A office space.