The video: Finland's capital is taking unusual steps to combat urban sprawl. Helsinki has gone underground, carving out a swimming complex, a shopping mall, and a church from the bedrock beneath its city streets. (View a CNN video below about the "Flintstones"-like initiative.) Underground Helsinki also has a hockey rink, "parking caverns," and many facilities used by the municipal government. Another subterranean operation is a big computer data center cooled not with electricity from a polluting power plant, but with chilly water channeled in from the ocean.

The reaction: Helsinki's "shadow city" is both "creepy and cool," says Jess Zimmerman at Grist. And it certainly seems pretty green to carve buildings into the earth, using the bedrock under the city as natural insulation from the elements. Indeed, a "fascinating project," says Brian Merchant at Treehugger. It's "a good way to maximize space and efficiency," leaving enough room above ground for the city to stay walkable and pleasant. Watch a CNN report: