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!).

In this view from 1911 we see the rugged topography of the site which is just one block wide but 13 blocks long. The Olmsted and Vaux design highlights the natural setting by creating two linear path systems, one broad and meandering and the other cutting through the Manhattan schist and hugging a massive buttressed retaining wall along the park’s western edge.