These are some of the coolest underground buildings in the world

Today, more than half of humanity lives in cities.

And we're only getting more urban - by 2050, at least two-thirds of the world's population will call cities its home.

Yet you can add only so many skyscrapers to squeeze in the flocking masses.

Building underground, however, is an option.

According to a 2013 report by the US National Research Council, exploring underground "may be the most successful way to encourage or support the redirection of urban development into sustainable pattern."

While experts figure out whether digging into the ground can solve the urban space problem, we take you to some of the world's most unique underground marvels.