While we've all heard of the Hoover Dam and Taj Mahal, some of the world's most mind-boggling megaprojects are still under construction. And these billion-dollar builds are set to become the priciest on the planet. From colossal dams and supersized airports to smart cities and futuristic rail networks, read on to discover the most expensive megaprojects happening right now, and how they're going to change our world. All dollar values in US dollars.