A vast undersea tunnel is being built that will change the road and rail map of Europe

The Fehmarnbelt Tunnel will carry two rail lines and a pair of two-lane highways under the Baltic Sea. Femern A/S via CNN Newsource

On the shores of the Baltic Sea, in the south of Denmark, a vast engineering marvel is taking shape — piece by giant concrete piece — that, when finished, will drastically redraw the road and rail maps of Europe.

The Fehmarnbelt tunnel may not have grabbed the popular imagination in the same way as the Channel Tunnel between Britain and France did more than 30 years ago, but this structure is just as impressive, if not more so.

Linking Denmark and Germany, the Fehmarnbelt will carry two-lane road highways under the water in both directions, plus two electrified rail lines — a multiple tube thoroughfare that will plunge beneath the waves of one of the world’s busiest shipping lanes.

At 18 kilometers (11.2 miles), it’s nowhere near as long as the 50-kilometer (31-mile) Channel Tunnel, but in many other ways it’s bigger. The project will, in fact, be the world’s longest road and rail tunnel, and the world’s longest immersed tunnel.

As an “immersed” tunnel, instead of being dug through solid land mass like the Channel Tunnel, the Fehmarnbelt is made using prefabricated concrete sections which are dropped into a trench dug on the seafloor, linked together, and then buried.

Last month, in February 2025, the first precast concrete tunnel sections, called elements, left the factory in Rødbyhavn, on the Danish side of the tunnel, completing Part One of a groundbreaking journey from land to sea that is scheduled for completion in 2029.

The 7.4 billion-euro (US$7.7 billion) project is a huge undertaking, in every sense.

Each section of the tunnel is colossal. These concrete structures measure 217 meters (712 feet) long, 42 meters wide and nine meters deep, weighing in at a crushing 73,000 tons — equivalent to 10 Eiffel Towers.

In all, 79 of these “standard” elements and 10 shorter “special” elements measuring 39 meters long (which will house the tunnel’s electrical installations) will be laid end-to-end, from Rødbyhavn on Lolland Island to Puttgarden on the German island of Fehmarn, creating the full tunnel up to 40 meters beneath the Baltic Sea.

Concrete proposal

Everything about the project is vast. The purpose-built factory is the world’s largest tunnel-making facility and combined with the work harbor covers an area of 220 hectares — the size of over 300 soccer pitches.

Three cavernous construction halls containing six production lines (five for standard elements, one for special elements) are currently working around the clock producing one 217-meter element every nine weeks.

Cast around dense cages of reinforcing steel, each concrete element consists of nine 24-meter-long segments. Once cured the element is pushed forward into the upper basin, a large staging area in front of the factory, where it undergoes quality checks and outfitting.

“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.”

Ballast tanks are fitted for stability and steel bulkheads are secured at either end of each section to make them watertight. Sealing the elements off like this enables them to float, despite their mammoth tonnage.

The first floating was successfully completed in early February when the upper basin was sealed off from the factory and slowly flooded with almost a billion litres of seawater. Once floated, the element was towed into the lower basin in preparation for its final voyage to the tunnel trench.

A date for the immersion of the first element hasn’t been finalized but preparatory work is in full swing, says Juchem.

“This is a very complex and also very weather-dependent process. We are currently testing the highly complex vessels that have been specially built for our project.”

Buoyed up

Enter “Ivy 1” and “Ivy 2,” the two immersion pontoons whose job it will be to make sure each element is safely lowered into place.

The pontoons will attach to either end of the element before a small fleet of tugboats will pull them into position above a 12-meter deep, 100-meter wide, gravel-lined trench completed last year.

The immersion will be overseen from a control room aboard the pontoon. Winches with steel wire totaling over 200 kilometers will be used to lower the element while the ballast tanks will be filled with water aiding a safe and steady descent to the seafloor.

Underwater cameras and bespoke positioning equipment will guide the element into place with a precision of up to 12 millimeters. Giant rubber gaskets and secondary seals engineered to last 120 years ensure the tunnel is watertight.

When the first immersion goes ahead later this year, Juchem estimates that it will take around 40 hours to complete, including transportation from the work harbor. The pontoon’s crew of 22 are undergoing intensive training.

“There will be no test run for the actual immersion,” Juchem says. “It must work the first time. We will not compromise on quality and safety. That is why we are taking the necessary time to ensure that we are perfectly prepared.”

Tunnel vision

Construction of the two tunnel portals at Rødbyhavn and Puttgarden is progressing. Both sites are a stone’s throw away from the ports which they will supersede, slashing journey times from 45 minutes by ferry to 10 minutes by car and seven minutes by train.

A traveler wanting to take the train from Hamburg to Copenhagen has a near five-hour journey at present but that time will be cut in half when the tunnel opens.

“The Fehmarnbelt tunnel will be a game-changer for tourism in Denmark and the wider Scandinavian region,” Mads Schreiner, International Market Director at VisitDenmark, told CNN.

“By significantly reducing travel time between Germany and Denmark, it will make our country more accessible than ever for visitors from central Europe. We expect to see a rise in self-drive tourism, weekend city breaks, and sustainable travel options such as train and cycling tourism.

“This new connection presents incredible opportunities for destinations in Eastern Denmark, as more travelers will explore beyond Copenhagen.”

The tunnel also comes with a challenge for Denmark to renew and expand facilities for the expected influx of new travelers, Schreiner says.

“It is crucial that we ensure visitors choose to stay and experience Denmark rather than just passing through. By investing in infrastructure and creating compelling visitor experiences, we can maximize the tourism potential of this historic project.”

Sustainable future

Inevitably with such a big infrastructure project, concerns have been raised over its environmental impact on marine and coastal habitats.

In November 2020, a federal court in Germany dismissed an appeal against the project’s planning approval over a range of claims including environmental concerns. And in 2022, an emergency application to halt dredging near Puttgarden was also rejected.

Femern A/S says it is committed to protecting the environment and where the impacts of construction are unavoidable they are creating new natural areas elsewhere.

For example, the 15 million cubic meters of seabed materials excavated from dredging has been deposited in new areas of reclaimed land in Lolland and Fehmarn.

“These sites are part of the natural environment that Femern A/S is establishing as a replacement for the areas affected by the project, and they will eventually develop into extensive wetlands with beach meadows, marshes and dry grassland,” Femern CEO Henrik Vincentsen wrote in the company’s 2023 Sustainability Report.

“Femern A/S aims to leave not just more, but better, nature than before the project started.”

Article written by Matthew Knight, CNN