 In the 1960s, Downtown Houston was a modest collection of mid-rise office structures, but has since grown into the third largest skyline in the United States.In 1960, the central business district had 10 million square feet (1,000,000 m²) of office space, increasing to about 16 million square feet (1,600,000 m²) in 1970.Downtown Houston was on the threshold of a boom in 1970 with 8.7 million square feet (870,000 m²) of office space planned or under construction and huge projects being launched by real estate developers.