On July 21, 1999, five commercial divers set out into the Deer Island outfall tunnel—a nine-and-a-half-mile dead end bored through the bedrock beneath Boston Harbor, with no light, no ventilation and no breathable air. Their job was to pull the safety plugs that stood between the finished tunnel and the sea. Two of them never came out.

Neil Swidey reconstructs how the largest public works project of its kind—the multibillion-dollar cleanup that turned one of America’s filthiest harbors into one of its cleanest—came down to a jury-rigged breathing system built around a gas blender designed for packaging food. Drawing on hundreds of interviews, depositions and internal memos, he traces the brinkmanship between contractor and agency that stripped the tunnel of its life support, the engineer whose improvised plan was allowed to stand unchallenged, and the long aftermath of investigation, litigation and trauma.

A story about ambition and blame, and about the blue-collar workers who are asked to turn other people’s dazzling plans into concrete reality.