New York’s geology is one of the worst for building tunnels—most of Manhattan was originally marshy, and the ground beneath lower Manhattan and much of Brooklyn is soft and filled with water. In places where it was never marshy, such as upper Manhattan, and many parts of the Bronx, extrusions of impossibly hard dolomite and other igneous rocks make for very slow digging. Under the Hudson and East rivers there are even more problems; the riverbeds are shifting masses of silt and mud, and reliable ground is found only very deep beneath the surface.

Despite these difficulties, New York City has more than 160 miles of tunnels for transportation—mostly subway tunnels, along with tunnels for cars, PATH trains, and other railroads. New techniques had to be pioneered for some of this work and mistakes often proved lethal. Twenty men were killed in 1880 when a digging shield failed in the first attempt to tunnel from New Jersey to Manhattan. The tunneling projects gave rise to an entire breed of construction worker, known as “sandhogs.” It also led to various technical innovations, such as the cut–and–cover method for subway tunnels and the development of large boring machines that allowed workers to reinforce the tunnel as the machine moved along, which helped give the sandhogs a modicum of safety.