Immersed Tube Technique
The immersed tube method, a version of the cut and cover technique, is a cheap and easy way to construct a tunnel under water -- and it's generally the fastest way.

Here's how it works:
Dredging machines cut a trench in the riverbed. Then one by one, preassembled, watertight tubes are floated out and lowered into place over the trench. Divers bolt each segment together to form the complete tunnel. The tunnel is then covered with a steel-and-concrete casing to prevent the tunnel from floating and to protect it from damage caused by passing ships. Underwater tunnels around the world, including Hong Kong's metro tunnel system and San Francisco's Bay Area Rapid Transit tunnel, were put together like this.

Start over		Check out another tool or technique