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.