Denmark wants to build a tunnel under the Fehmarn Belt, the 18-kilometer strait between Germany and Denmark, at a cost of over 5 billion euros. The rail journey from Hamburg to Copenhagen, which currently takes around four and a half hours, would be shortened by one hour.

Tunnel construction would begin in 2014 for a planned opening in 2020. The transit passage would consist of three tunnels: two containing a four-lane highway and a third one with two train tracks. Currently, the stretch between Rødby in Denmark and Puttgarden in Germany, north of Hamburg, has only been served by ferry.

What Denmark plans to build across this 18-kilometer (11-mile) stretch of water is no less than the world's longest underwater tunnel that uses the so-called immersed tube method. The individual elements would be built on land and then sunk onto the sea bed. The concrete pipes would be 200 meters long and weigh around 70,000 tons each. Each pipe has the proportions and weight of a supertanker ship.