New York City's Second Avenue subway line was the greatest project the city never built -- until 2007, almost 90 years after its initial proposal, when the city finally broke ground on it. When completed, the $4.45 billion project will provide New Yorkers with two more miles of commutable subway tunnels on Manhattan's East Side.
Photos released on the MTA's Flickr page show a few pieces of the new route in-progress--and just how hard it is to dig a miles-long tunnel under a densely-populated island city.
Five amazing, clean technologies that will set us free, in this month's energy-focused issue. Also: how to build a better bomb detector, the robotic toys that are raising your children, a human catapult, the world's smallest arcade, and much more.