Subterranean Sprawl: Cities Expand Underground

"The world's cities may be overcrowded, and the suburbs that surround them may be choked by unchecked sprawl, but there's plenty of undeveloped space. It's right at your feet: underground...The cost to burrow down is dropping, while the price (and hassle) of erecting a skyscraper in a dense urban area just keeps rising...Over the next 10 years these techniques will be used to hollow out space beneath the world's great cities...Maglev trains running through depressurized tunnels are the logical successor to airplanes, at least between large cities...Some cities have already started to expand underground. Both Toronto and Montreal have extensive downtown retail complexes below the surface...Likewise, Kansas City has 20 million square feet of light industrial space sited in old quarries beneath the city."