Fehmarnbelt Tunnel 18 KM, 89 Giant Elements | How Denmark Built the World’s Longest Immersed Tunnel

How do engineers build an 18-kilometre tunnel beneath the sea connecting Denmark and Germany? The Fehmarnbelt Immersed Tunnel is one of Europe's most ambitious infrastructure projects. Instead of boring through the seabed with conventional tunnel boring machines, engineers are constructing enormous concrete tunnel elements on land, transporting them to sea, and carefully lowering them into a massive trench excavated across the Fehmarnbelt. The finished tunnel will connect Lolland in Denmark with Fehmarn in Germany, carrying both high-speed railway and motorway traffic beneath the Baltic Sea. The tunnel is designed with two railway tracks and four motorway lanes. 🏗️ How Is the Fehmarnbelt Tunnel Built? 1. Excavating the seabed Before the tunnel elements can be installed, engineers excavate an approximately 18-kilometre-long trench across the seabed. Around 15 million cubic metres of material are removed, creating the foundation for the tunnel. 2. Building a giant tunnel factory Near Rødbyhavn, Denmark, a purpose-built factory produces the massive concrete sections. The production system includes five production lines for standard elements. Each standard element is made from nine concrete segments assembled into one enormous structure. 3. Creating 89 massive tunnel elements The tunnel consists of 89 elements: 79 standard elements and 10 special elements. Each standard element is approximately 217 metres long and weighs more than 73,000 tonnes. Imagine moving a structure the length of more than two football fields while it is floating on the sea. 4. Transporting the elements to sea Once completed, each giant concrete element is moved from the factory through the specially constructed work harbour. Tugboats then tow the massive structure toward its position above the underwater trench. 5. The most dangerous step — controlled immersion This is where the project becomes extraordinary. Special immersion vessels position each enormous element with extreme precision before controlled flooding and lowering operations bring it down toward the seabed. In May 2026, the first 217-metre-long, 73,500-tonne element was successfully immersed. The operation took roughly 14 hours from the beginning of immersion until the element reached its precise position. 6. Connecting the tunnel underwater After reaching the seabed, the element is aligned and connected to the previous section. Gravel and sand are then placed around the structure to secure it, followed by a protective rock layer above the tunnel. The process is repeated element by element until the entire 18-kilometre underwater crossing is completed. 7. Installing the railway and motorway systems Inside the completed tunnel, engineers install railway tracks, electrical systems, ventilation, lighting, emergency systems and road infrastructure. When finished, the tunnel will provide a much faster connection between Scandinavia and mainland Europe. The planned journey through the tunnel is approximately 7 minutes by train or 10 minutes by car.