The man-made cave is composed of 28 pre-cast concrete sections, which fit together using a tongue-and-groove system. They're modified versions of a standard concrete culvert used under roads and highways. Most of them weigh between 17 and 21 tons apiece.
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.