Imagine the frustration faced for so many years by Eric W. Jordan and his colleagues. They could take a pretty good guess at what lay hundreds of feet beneath the macadam-sealed surface of New York City’s streets. They just had no way of knowing for sure.

But the last 10 years or so have been a boon to Mr. Jordan and his fellow geologists; mammoth subterranean excavations for the city’s Third Water Tunnel, the Second Avenue Subway and the Long Island Rail Road’s East Side Access Project have enabled them to see for themselves the rock formations and faults that they had only been able to imagine, undergirding Manhattan.

As a child, Mr. Jordan played with blocks, not rocks, but he had an epiphany when he took an environmental geology course at Western State College of Colorado, now Western State Colorado University, in Gunnison. These days, he is truly in his element.

Mr. Jordan, 50, works for Parsons, an engineering firm. He has spent countless hours underground, consulting for the Metropolitan Transportation Authority and the city’s Department of Environmental Protection on all three multibillion-dollar tunnels, which individually are bringing forth an alternative source of water from the Catskills, the first phase of a new subway on the Upper East Side and a Park Avenue tunnel linking the Long Island Rail Road from Queens to Grand Central Terminal.