Constructing New York's First Subway
Construction on New York's Interborough Rapid Transit, or IRT, began in 1900. William Parsons, the project’s chief engineer, used a variety of techniques for underwater digging. Sometimes hydraulic shields were used; other times methods were improvised, like digging a trench in the riverbed, then sinking the tunnel roof down on top of it. At least 7,700 men would be needed to build the IRT.