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 ...

The 2,625 ft eight-lane immersed tube tunnel, which works out at about £880,000 per foot, should be able to withstand earthquakes, water pressure and heavy traffic loads when it opens in 2030.

Once operational, it will become the longest immersed tunnel in the world, diving 40 meters below sea level. Currently, travelers and cargo rely on ferries to cross the Fehmarnbelt Strait ...