Underground Cities: The Next Frontier Might Be Underneath Your Feet

In 1931, construction workers added the final nail to the Empire State Building in New York. At 1,250 feet high, it was the tallest building in the world, and remained so until the 110-story north tower of the World Trade Center finalized in 1972

Then came Chicago's Sears (now Willis) Tower, followed by the Petronas Twin Towers in Kuala Lumpur and the Taipei 101 in Taiwan. In 2010, Dubai completed construction on the massive, 2,722-foot-high Burj Khalifa.

The 20th century was inarguably the era of the skyscraper. Cities across the world, out of necessity and sheer showmanship, expanded up, up, up. But the 21st century is seeing a new trend of going underground instead. Urban areas such as Helsinki and Paris are looking to expand below the surface for resource, retail and travel purposes. Read more...