A massive engineering project is taking shape on the southern coast of Denmark, where an 18-kilometer underwater tunnel will soon transform European road and rail networks.
The Fehmarnbelt Tunnel may not have captured public attention like the Channel Tunnel did over 30 years ago, but it is just as remarkable—if not more according to CNN.
This 18-kilometer immersed tunnel between Denmark and Germany will set new engineering milestones, enhancing European connectivity in ways just as profound.
The Fehmarnbelt Tunnel will feature a two-lane highway in each direction alongside two electrified rail tracks, creating a multi-tube passage beneath one of the world’s busiest shipping lanes.
At 11.2 miles, it’s shorter than the 31-mile Channel Tunnel, but in many ways, it’s even more ambitious.
Once completed, it will be both the world’s longest road and rail tunnel and the longest immersed tunnel ever built.
Unlike the Channel Tunnel, which was drilled through solid rock, the Fehmarnbelt Tunnel is an immersed tunnel, built using prefabricated concrete sections. These sections are lowered into a trench on the seafloor, linked together, and then buried.
In February 2025, the first precast concrete tunnel elements left the factory in Rødbyhavn, Denmark, marking a major milestone in the project.
This is the first step in a groundbreaking journey that will culminate in 2029, when the tunnel is set to open.
The €7.4 billion ($7.7 billion) Fehmarnbelt Tunnel is a massive engineering feat in every way.
Each precast concrete section is enormous, measuring 217 meters (712 feet) long, 42 meters wide, and 9 meters deep.
Weighing 73,000 tons—the equivalent of 10 Eiffel Towers—these colossal structures will be carefully lowered and connected on the seabed to form the world’s longest immersed tunnel.
“We try to install as much of the equipment as possible that will later be needed in the finished tunnel,” Denise Juchem, a spokesperson for Femern A/S, the state-owned Danish company in charge of the project, told CNN.
“This saves time later and reduces the logistical challenges for the work in the immersed tunnel. In addition, monitoring equipment is installed so that we can check the interior of the element at any time during transport and immersion.”
However, as with any major infrastructure project, the Fehmarnbelt Tunnel has faced concerns over its environmental impact on marine and coastal ecosystems.
Legal challenges have not stopped progress. In November 2020, a German federal court dismissed an appeal against the project’s approval, rejecting claims related to environmental risks. Again, in 2022, an emergency application to halt dredging near Puttgarden was also denied, allowing construction to continue.
