BUILDING ANY TUNNEL CAN PRESENT A HOST OF DIFficulties, from getting soil out and construction materials in to performing precision alignment deep inside the earth. When the tunneling takes place underwater, a whole new set of obstacles arises.

Starting early in this century, however, advances in structural steel and reinforced concrete technology have allowed the development, mostly in the United States, of the immersed-tunnel method, in which prebuilt sections the length of a city block are floated into place and sunk to their proper position. The process is similar (on a grand scale) to the method used on land in which builders simply dig a trench, lay pipe, and cover it up. The need to keep everything in place beneath dozens of feet of constantly shifting water and mud complicates the underwater version greatly, but with a carefully crafted design and an experienced construction team, it is often safer, cheaper, and more reliable than any other way. In my four decades as a civil engineer, I have worked on a number of tunnels built by the immersed method, and my growing familiarity with the technique has only deepened my appreciation of the planning, coordination, and teamwork that go into it.

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