When you’re planning to build some of the world’s most spectacular and challenging tunnels, you need to know ahead of time what you’ll be digging through. Is it soil, sand, or stone? Is the stone strong and secure or friable and likely to collapse? To know what the bedrock is like, you need a drill rig to dig it out from hundreds of feet below the surface. The drill extracts geological core borings, cylindrical samples that provide a cross-section of subsurface conditions and let engineers know whether the area is good for tunneling and what construction methods should be used. The drill bits are tubular and loaded with shot or embedded with diamonds to cut around the core, like a drinking straw plunging into a milkshake (except really, really hard!).

Washington Bridge, ca. 1940. As pre-consolidation New York City grew, it gradually absorbed the northern Manhattan neighborhoods of Harlem, Yorkville, Manhattanville, and Carmansville. This expansion and the 1870s development of housing in newly-laid-out Washington Heights promoted the idea for a vehicular traffic link between upper Manhattan and The Bronx, where population growth was still minimal. Washington Bridge (originally called the Harlem River and then the Manhattan Bridge) opened in 1889, setting a record completion time of just under two years, and making the relatively rural Bronx approachable for development. Although still part of Westchester, a status that changed with consolidation, The Bronx was newly accessible to Manhattan by light rail surface transit. Eventually, with the opening of the George Washington Bridge, the Washington Bridge became part of the interstate system carrying U.S. Route 1 from Maine to Florida. Period guidebooks recommended the center of the bridge, 155 feet above the river, as a site from which to enjoy a superb view that encompassed the Harlem River, Long Island Sound, Fort George, Spuyten Duyvil, and Kingsbridge. Fifteen-foot-wide walkways on either side of the roadway provided amply for those who chose to enjoy the view on foot.

First navigated by the Lenape, and then owned by the Dutch, the "Harlaem" River has long been a waterway important to the history of New York. This story highlights some of the structures built over the Harlem to connect Manhattan to the broader world.