Boston's Central Artery Tunnel Project -- commonly known as the Big Dig -- is one of the largest urban highway projects ever constructed.

It consists of a 7.8-mile tunnel under downtown Boston, two bridges, and four highway interchanges. What made the project so complicated -- and expensive -- was that it was built under a functioning city. The major elevated highway it replaced needed to stay open throughout the construction process, as did subway tubes running through the tunnel's path. To overcome this, engineers had to build temporary supporting bridges underneath existing infrastructure, and in some cases literally freeze the soil so the city would not collapse on itself.

Cost overruns and delays were legendary. When it was completed in 2006, the final price tag of over $15 billion was nearly three times the 1991 estimate, when construction first began. But its effect on the city -- and urban planing in general -- was transformative.

"It reconnected the city, and showed how you provide big infrastructure without taking people's homes," said David Luberoff, an urban land use consultant and co-author of Mega-Projects. "But it was expensive and complicated."