Inspired by the need to build in a variety of conditions, tunnel builders have, over the years, developed tremendous innovations in techniques, tools, and safety provisions. Tunnels must be designed first and foremost to withstand pressure -- from above, from the sides, and sometimes from below. For this reason, nearly all are arch-shaped or tubular. The weight of surrounding rock, earth, or water makes tunnels built in these naturally strong shapes even stronger.

The amount of time an excavated passage will stand unsupported -- called stand-up time -- is an important consideration in tunneling. Solid rock, though hard to dig through and remove, has excellent stand-up time and may require no structural support during or after excavation. Loose gravel and wetter soils, however, such as the silt and mud found under water, have very little, if any, stand-up time. When the ground cannot stand up on its own, tunnel builders must construct a tube of reinforced concrete or other strong material as they dig the hole. This ensures the permanence and safety of the tunnel.

The 32-mile Channel Tunnel, a rail tunnel constructed under the English Channel, is the longest underground tunnel ever built. French and British tunneling teams employed specially designed tunnel boring machines (TBMs) to complete the task. A TBM's rotating cutting head loosens material from an earth or rock face and deposits it onto an outgoing conveyor belt. A second conveyor belt carries in lining material -- in this case thick, concrete tiles -- to stabilize the tunnel's inside walls. Working in this way, each Channel Tunnel team left a completely finished cylindrical passage as it advanced toward a central meeting point. The Channel Tunnel project proved to be a remarkable technical achievement, even more so considering the British team encountered weak, wet chalk in places despite extensive ground samples that suggested they would be digging through a soft, dry layer made of chalk and clay.