Denmark and Germany are building an 11-mile tunnel beneath the Baltic Sea using massive 73,000-ton concrete blocks. The Fehmarnbelt Tunnel will connect Lolland in Denmark with Fehmarn in Germany, replacing a 45-minute ferry crossing with a 10-minute drive or a two-hour faster train trip between Copenhagen and Hamburg.

When completed in 2029, the €7.4 billion project will be the world's longest immersed road-and-rail tunnel. Engineers will assemble it from 89 prefabricated concrete sections, each 217 meters long and weighing up to 73,500 tons. The construction method involves floating these giant blocks into position and lowering them into a seabed trench.

Denmark is financing most of the project through tolls, with the European Union contributing over €1 billion as part of its Connecting Europe Facility. The tunnel forms a critical link in the Scandinavian–Mediterranean transport corridor, improving trade routes and reducing congestion on ferry services between northern Europe and central Europe.