The world’s longest undersea tunnel will be built in Europe. Specifically, it will link Denmark and Germany, proving to be an ideal solution for better connecting Scandinavia with the rest of the Old Continent. Many experts have dubbed the Fehmarnbelt the history-making tunnel, both for its size and for the construction method used, which makes it one of the most ambitious engineering projects of modern times.

The numbers

If numbers are what make a project great, then this undersea tunnel is certainly an infrastructure to watch closely. At 18.2 km long and 40 metres deep, it will cross the Baltic Sea, cutting travel times and distances compared to the current 160 km connecting the Danish shore at Rodby with the German shore at Puttgarden (currently a 45-minute ferry ride).

The Fehmarnbelt underground tunnel will consist of five lanes: two for cars travelling on the motorway, two for electrified trains and a central lane dedicated to services and safety measures. Cars will travel through the tunnel in about 10 minutes at speeds of up to 110 km/h, while trains will reach 200 km/h and take 7 minutes to complete the crossing.

The Danish desire: to emerge from isolation

These are important figures because they will enable faster transfers, reducing, for example, the road distance between Copenhagen and Hamburg; currently, the journey takes five hours, but thanks to the tunnel, it will take only two and a half hours. The shortcut will bring environmental benefits for both freight and passenger transport, as the 160 km reduction in distance will cut carbon emissions.

The project is largely funded by Denmark, reflecting the country’s desire to reduce its geographical isolation and open up more to the rest of Europe. This goal will be achieved by improving transport links. Of the €7.4 billion earmarked for the tunnel, €1.3 billion will come from the European Commission, with a smaller contribution from Germany and the rest from Copenhagen.

How it will be built

Originally, a bridge was planned, but strong winds in the area meant that an alternative had to be found for safety reasons to avoid the risk of ships crashing into the bridges.

This led to the idea of an undersea tunnel, although unlike other tunnels, which are built by excavating the rock, the Fehmarnbelt will be constructed on the surface and then submerged on the seabed.

To be precise, there will be 90 pieces assembled; these are individual concrete elements, 217 metres long and 42 metres wide, with a total weight of 73,500 tonnes, which will be combined with each other, replicating what happens with Lego.

Building a tunnel is like making a smartphone

This is no coincidence, considering that the world’s most famous building blocks were invented in Denmark. Once joined together, these enormous elements will be lowered into a deep trench using underwater cameras and GPS equipment to align them with a margin of error of 15 millimetres.

‘More than a construction project, we are operating as if we were making an iPhone,’ commented Mikkel Hemmingsen, CEO of Sund & Bælt, one of the companies involved in the work. This process has its advantages because, the executive added, if one element fails, it can be easily replaced.

The work is expected to be completed in 2029 when the Fehmarnbelt should be crossed by more than 100 trains and around 12,000 cars every day. These will help to repay the costs of the project through tolls over the next 40 years.