The human race has been making its mark on the planet since time immemorial, embarking on ever more ambitious, and expensive, constructions. The costliest run into tens and even hundreds of billions of dollars, and range from spectacular new cities to immense road and rail networks. From China's Great Wall to the US highway system, click or scroll through the 20 most expensive completed megaprojects of all time, adjusted for inflation into today's money, and get ready for your jaw to drop.