Boston’s Central Artery/Tunnel Project, the Big Dig, replaced an elevated six-lane highway built through downtown in the 1950s, a green steel structure that displaced 20 percent of North End businesses and was mocked as Boston’s other Green Monster. Designed for 75,000 vehicles a day, it carried 190,000 by the 1990s, and planners predicted 16-hour jams by 2010. After protests in the 1970s killed the Inner Belt bypass, the only option left was to go underground. Congress overrode President Ronald Reagan’s veto to fund it, work began in 1991, and the main phases were finished around 2007. Crews dug slurry walls 120 feet deep through landfill, shipwrecks, and old foundations while keeping the old highway open above them, and froze the ground with chilled brine to tunnel beneath South Station’s 400 daily trains.

Management by the Bechtel and Parsons Brinckerhoff consortium merged with the state turnpike authority, and the estimate of $2.8 billion grew to $14.6 billion, over $21 billion with interest. Leaks hidden since 2001 were exposed by MIT researchers, and in 2004 harbor water burst into a tunnel. In 2005 state police raided Aggregate Industries for mixing rejected concrete into fresh loads and forging delivery times. In July 2006 concrete ceiling panels held up with a short-term epoxy fell on a car and killed Milena Del Valle. The project cut vehicle hours of travel by 62 percent and created the Rose Kennedy Greenway, but induced demand pushed congestion to the suburbs.

• The rat mitigation program needed before crews could disturb the city’s underground
• How bentonite clay held slurry trenches open until concrete pumped from below pushed it out
• The Zakim Bunker Hill Bridge by Christian Menn, born from a lawsuit by Cambridge over ugly designs
• Square-edged Ginsu guardrails and a $54 million fix for 25,000 light fixtures corroded by galvanic reaction
• Abandoned transit promises including the North-South Rail Link, the Red-Blue connector, and the Arborway streetcar