In 1980s Boston, traffic on the city's Central Artery lasted 10 hours a day and cost the local economy about $500 million [source: MTA]. In response, the Central Artery/Tunnel Project -- or Big Dig -- was launched to replace the six-lane Central Artery with an underground road with eight to ten lanes. The project, one of the most difficult infrastructure projects ever attempted, also involved the construction of several other major bridges, roads and tunnels, some of them going under Boston Harbor.
Construction began near the end of 1991 and finished on Dec. 31, 2007. The original cost estimate was $2.6 billion, eventually rising to $14.8 billion by the time the project was done [source: LeBlanc]. But with the interest due on borrowed funds -- which will be paid through 2038 -- the Big Dig will eventually cost $22 billion [source: Murphy].
The city practiced a process it called mitigation to minimize harm to residents' daily lives and commutes. But mitigation also contributed to the project's massive cost overruns and didn't always work. Some businesses suffered because of the rampant construction.
There were other setbacks through flaws in design or construction, the deaths of four workers and one motorist, criminal charges against some companies and numerous lawsuits. But many aspects of the project's work -- which employed as many as 5,000 workers on some days -- have been called marvels of engineering [source: LeBlanc]. And the Big Dig has reunified the city, improved traffic flow greatly, opened up new parts of the city to development and added green space.
To see more Top 10s and to learn about other insanely ambitious construction projects, look over the links on the next page.