Construction of the world's longest immersed-tube tunnel is projected to cost $7 billion, with a completion date of 2021. The Fehmarn Belt Fixed Link will run between Germany and Denmark. Immersed-tube tunnels are good alternatives to bridges under certain conditions, said Jonathan Baber, co-author of "Immersed Tunnels" and Mott MacDonald project director. "While bridges are generally cheaper to build, immersed-tube tunnels are usually located at sites where bridges would not be possible -- either soil conditions are poor, or there is constricted space on the sides of the waterway, or there is a high clearance required for navigation by ships," Baber said.

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