Deep below ground level in Shanghai, workers on a flood-lit construction rig carefully install massive concrete wall sections for a new subway tunnel. Meter by meter, they are making the world's longest metro system even longer.

The future Line 14 is part of plans to extend the underground subway network in Shanghai by another 35 percent of its current size. By the time the project concludes in 2020, the network will span 830 kilometers. That's more than the distance from New York to Chicago.