On the shores of the Baltic Sea, in the south of Denmark, a vast engineering marvel is taking shape.
The Fehmarnbelt tunnel may not have captured 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 km (11.2 miles), it’s nowhere near as long as the 50 km (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 huge prefabricated concrete sections which are dropped into a trench dug on the seafloor, linked together, and then buried.
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.
Last month, in February 2025, the first precast concrete tunnel sections 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 ($7.7 billion) project is a huge undertaking, in every sense.
A traveller 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.
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