How do you build an underwater tunnel?

When digging into any extraordinary construction project, a few questions immediately poke their heads out of the sand: Which proposal ranks as the biggest, the deepest or the most hazardous to build? With underwater tunnels, these questions defy simple answers. Cities and countries constantly commission new projects. As for vital stats, the devil is between the details and the deep, blue sea.

For example, the Seikan Tunnel connecting the Japanese islands of Honshu and Hokkaido currently holds the record for the longest and the deepest underwater rail tunnel. Japan began planning it after a 1954 typhoon sank five ferry boats in the dangerous Tsugaru Strait, killing 1,430 people [sources: WGBH].

Completed in 1988, the Seikan Tunnel stretches 33.5 miles (54 kilometers) and reaches a depth of 787 feet (240 meters), but its 14.5-mile (23.3-kilometer) undersea portion is dwarfed by that of the Channel Tunnel, or Chunnel, between the United Kingdom and France. Finished in 1994, the Chunnel's underwater portion accounts for 24 of its 31 total miles (38.6 of 50 kilometers) but plunges only 246 feet (75 meters) deep [sources: ASCE; Chan; Wise].

As far as the Turks are concerned, both tunnels are all wet compared to their $3.3-billion Marmaray Tunnel finally open to the public in 2013. Its 8.25 miles (13.2 kilometers) of railroad passage -- including a 4,600-foot (1,400-meter) stretch across the Bosporus seafloor -- connect Istanbul's Asian and European halves, making it the first rail tunnel to connect two continents [sources: Sweeney; Wise].

What's so great about a sub-mile undersea tunnel compared to the multi-mile Seikan and Channel tunnels? It's a difference of approach: Whereas its predecessors respectively blasted and bored passages through solid rock, the Marmaray Tunnel was assembled, piece by piece, in a trench along the Bosporus' bottom, which makes it the longest and deepest immersion tunnel ever built. Engineers chose this solution, which employs preassembled sections connected by thick, flexible, rubber-reinforced steel plates, to better contend with regional seismic activity [sources: JR-Hokkaido; Sweeney; Wise].

For a time, cultural and historical artifacts found throughout Istanbul's old city slowed progress on the Marmaray Tunnel excavation, so the 2.2-mile (3.6-kilometer) Øresund Tunnel connecting Sweden and Denmark remained the largest immersed-tube tunnel ever built. Contractors constructed it from 20 elements measuring 577 feet (176 meters) apiece, each assembled from eight smaller, 72-foot (22-meter) sections [sources: Landler; Marmaray Project; PERI GmbH; Sweeney].

Between immersed tunnels like Marmaray and Øresund, and bored tunnels like the Chunnel, we've just about covered the waterfront. But let's delve a little deeper into each and examine another tunneling method used since the early 19th century.

The oldest approach to tunneling underwater without diverting the waters above is known as a tunneling shield, and engineers still use it today.

Shields solve a common but vexing problem, namely, how to dig a long tunnel through soft earth, especially underwater, without its leading edge collapsing [sources: Assignment Discovery; Encyclopaedia Britannica; Browne; Hewitt].

To get a sense of how a shield works, imagine a lidless coffee tin with a sharpened bottom that sports several large holes. Now, gripping the open end, push the tin, bottom first, into some soft earth and watch how the dirt squeezes through the openings. On the scale of a real shield, several humans (nicknamed "muckers" and "sandhogs") would stand inside compartments within the "can" and remove the clay or sand as the shield advanced. Hydraulic jacks would gradually move the shield forward, while crews behind it installed metal supporting rings, then lined them with concrete or masonry [sources: Assignment Discovery]; Encyclopaedia Britannica; Browne].

In order to hold back water seepage from tunnel walls, the front of the tunnel or shield is sometimes pressurized with compressed air. Workers, who can only withstand short periods in such conditions, must pass through one or more airlocks and take precautions against pressure-related sicknesses [sources: Hewitt; Port Authority].

Shields are still used, especially when installing utility conduits or water and sewage pipes. Although labor intensive, they cost only a fraction as much as their mammoth cousins, the tunnel boring machines (TBMs) [sources: Assignment Discovery; Encyclopaedia Britannica; WGBH].

A TBM is a multistory-tall engine of destruction capable of chewing through solid rock. At its front spins a cutting head, a giant wheel that bristles with rock-breaking disks and incorporates a system of scoops to lift pummeled rock and drop it onto an outbound conveyor belt. Behind the cutting head swings an erector, a rotating assembly that builds the tunnel lining in the TBM's wake. In some large projects, like the Chunnel, separate TBMs will begin on opposite ends and drill toward a central point, using sophisticated surveying methods to keep them on course [sources: Assignment Discovery; Coleman et al.; WGBH].

Drilling through solid rock creates a largely self-supporting tunnel, and TBMs drive forward quickly and relentlessly (some Chunnel machines could bore 250 feet, or 76 meters, per day). On the down side, TBMs break down more often than a used Jaguar and deal poorly with worn, sheared or highly jointed rock -- so they aren't as fast as they're cracked up to be [sources: WGBH; WGBH].

Luckily, TBMs and shields aren't the only game in town.

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.

The title for the longest underwater tunnel has been hard fought. But, Norway has big plans to put the debate to rest for good with the the Norwegian Coastal Highway Tunnel [source: TunnelBuilder].

This colossal vehicular tunnel is expected to be completed in the 2030s and is set to redefine the benchmarks of underwater tunneling. Norway, with its rugged coastline and deep fjords, has always posed a challenge for connectivity. The Coastal Highway Tunnel aims to replace ferry services and bridge the physical gaps, facilitating a continuous route along the western coast.

Envisioned to be over 27 kilometers long, the tunnel will surpass current records, diving under the Norwegian Sea, and reaching depths of approximately 390 meters below sea level, it represents the pinnacle of underwater tunnel construction.

The planning and design of this monumental structure required groundbreaking innovation. Unlike traditional bored or immersed-tube tunnels, the Norwegian engineers are pioneering the use of submerged floating tunnels, a first of its kind.

These tunnels, consisting of tube sections that float underwater, supported by pontoons and anchored firmly to the seabed, offer a novel solution to the challenges of deep-sea tunneling. This method not only circumvents the difficulties of drilling through the tough subsea bedrock but also withstands the pressure and the harsh conditions of the Norwegian Sea.

If all goes according to plan, the Coastal Highway Tunnel may one day stand as the longest tunnel in the world. Its completion will mark a new era for Norway and for the world of civil engineering, as it stands as a silent, sturdy sentinel beneath the waves, a submerged triumph over the tumultuous sea.