Worlds Awesomest Tunnels
I could sit here and tell you all about the cost overruns and political skullduggery and interminable delays surrounding this massive feat of infrastructure, but that d take forever. Instead just consider this: the team in charge of the Big Dig took an entire interstate highway (I 93) and put it underground. They opened up an entire section of Boston that had previously been demarcated by massive concrete barriers essentially taking a Robert Moses and making it a Jane Jacobs. It may have been a 20 year headache, but at the end of the day the Big Dig is one of the most ambitious engineering projects in American history, and it worked.