A record-breaking tunnel is currently being constructed under the Baltic Sea, connecting Denmark and Germany in a project that promises to revolutionize transport links between Scandinavia and central Europe. Known as the Fehmarnbelt Tunnel, the 18km (11-mile) structure will become the world’s longest immersed road and rail tunnel when completed.
Located at the northern entrance on Denmark’s Lolland island, the construction site stretches over 500 hectares and includes a factory to manufacture the enormous tunnel sections, known as "elements." Each element, cast in reinforced concrete, measures 217 metres long and weighs more than 73,000 tonnes. Unlike traditional underwater tunnels like the Channel Tunnel, which are bored through bedrock, the Fehmarnbelt Tunnel’s segments will be placed directly onto the seabed and linked together.
Henrik Vincentsen, CEO of Femern, the Danish state-owned company overseeing the project, describes the operation as a record-setting engineering feat. “Immersed tunnels have been built before, but never on this scale,” he says.
The tunnel is designed with five parallel tubes: two for road traffic, two for railways, and one for maintenance and emergencies. Construction manager Anders Gert Wede explained that the heavy sections, once completed, are towed out to sea, positioned with precision using underwater cameras and GPS, and then slowly immersed into a pre-dug trench with accuracy down to 15 millimetres.
Funded primarily by Denmark with €1.3 billion support from the European Commission, the €7.4 billion project is one of the largest infrastructure investments in northern Europe. The tunnel will replace the current 45-minute ferry crossing between Rødbyhavn and Puttgarten with a ten-minute car journey or a seven-minute train ride. Additionally, it will halve train travel time between Hamburg and Copenhagen from five to two-and-a-half hours.
Professor Per Goltermann of the Technical University of Denmark noted that a bridge was initially considered, but potential disruptions from high winds and the risk of ship collisions made a tunnel the safer, more cost-effective choice.
Despite initial legal challenges from environmental groups concerned about the tunnel’s impact on Baltic marine life, the project received final approval in 2020. Femern has undertaken environmental mitigation efforts, including the creation of a 300-hectare wetland nature reserve using dredged material.
The tunnel is expected to serve more than 100 trains and 12,000 cars daily once it opens in 2029. Revenues from toll fees will repay the state-backed loans over an estimated 40 years. Local communities, particularly in economically challenged Lolland, are hopeful that the tunnel will drive growth in jobs, tourism, and business.
