More than halfof humanitylives in cities, and the world's urban population is going to explode in the coming decades.By 2050, at leasttwo-thirds of the planetary populationwill call cities home. All of those people will need places to live and work, andyou can addonly so manyskyscrapers to squeeze in the flocking masses.One solution: building underground.Lots of cities already have linked-up subterranean spaces, but as urban areasbecome more crammed, you can expect these spaces to multiply.According to a2013report bythe US National Research Council, exploringunderground "may be the most successful way to encourage or support the redirection of urban development into sustainable pattern."While expertsfigure out whether digging into the groundcan solve the urbanspace problem, we take you to someoftheworld's most unique underground marvels in cities and beyond.Built into a rock, the Temppeliaukio Church in Helsinki gets ample sunlight through a glazed dome. Those rough rock walls were left untouched by the designers for a reason: naturally great acoustics make the church a perfect venue for concerts.In this southern Tunisia village, locals live in traditional "troglodyte' dwellings ' cave houses ' created by scraping away rocks. There is even a subterranean hotel.This house on the Greek island of Antiparos sits right where two slopes meet. Two long stone walls bridge the hills, allowing the house to naturally blend in the space.See the rest of the story at Business Insider NOW WATCH: What Barbie would look like if she had the body of an average American teenager Click here to read full news..