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How do you build an underwater tunnel?

Letting It Sink In

Constructing a steel-and-masonry support while simultaneously digging through soft earth or solid rock is no picnic, but trying to hold back a sea while underwater is something not even Moses would have attempted. Fortunately, thanks to American engineer W.J. Wilgus and his invention, the sunken- or immersed-tube tunnel (ITT), we don't have to [source: Lane].

ITTs aren't bored through rock or soil; they are assembled on-site from football-field-sized, prefabricated pieces. Wilgus pioneered the technique when he built the Detroit River railroad tunnel (1906-10) connecting Detroit, Mich., to Windsor, Ont., and they've been the go-to technique for vehicle tunnels ever since. Indeed, more than 100 such tunnels were built in the 20th century alone [sources: Lane; Extreme Engineering; Marmaray Project].

To make each tunnel segment, workers assemble 30,000 tons of steel and concrete -- enough for a 10-story apartment building -- in a massive mold, then allow the concrete to cure for nearly a month. The molds contain the tunnel's floor, walls and ceiling, and are initially capped at the ends to keep them watertight as they are transported out to sea. Immersion pontoons, large ships resembling a cross between a gantry crane and a pontoon boat, do the hauling [sources: Lane; Extreme Engineering; Marmaray Project].

Once over the pre-dug sea trench, each tunnel section is flooded enough to allow it to sink. A crane slowly lowers the section into position while divers guide it precisely to its GPS coordinates. As each new section connects to its predecessor, a massive rubber piece on its end squeezes and distends to establish a seal. Crews then remove the bulkhead seals and pump out the remaining water. Once the entire tunnel is built, it is buried under backfill and possibly covered with rock armor [sources: Lane; Extreme Engineering; Marmaray Project].

Immersed-tube construction can delve deeper than other approaches because the technique does not require compressed air to hold water at bay. Crews can therefore work longer in them and under more tolerable conditions. Moreover, an ITT can take any form, unlike a bored tunnel, which follows the shape of its shield or TBM. However, because ITTs only make up the seafloor or riverbed portion of a tunnel system, they require other tunneling methods to bore their land-based entrances and exits [sources: Lane; Marmaray Project; WGBH]. In underwater tunneling, as in life, it takes all kinds.