A 712-foot concrete box weighing 81,000 tons was lowered onto the Baltic seabed; it is one piece of an 11-mile-long tunnel to Germany

Where a strait is too busy or too deep for a bridge, engineers can build a tunnel in pieces instead of boring through the ground. Concrete hollow sections are built in a dry dock or shore-based factory, sealed at both ends, floated to the site and lowered into a trench dug in the seabed and then buried under backfill. Every section should float, sink in a controlled manner, settle on a prepared base and connect to its neighboring section within narrow tolerances and the entire process is weather and sea condition-dependent. Thus, the placement of each section is considered an important event, especially the first one for a newly built tunnel.The first of 89 elements left the Rødbyhavn tunnel factoryThe Czech Republic’s entry in the ITA Member Nation Activity Reports 2020 describes the Fehmarnbelt Fixed Link as an 18 km immersed tunnel made up of 89 concrete elements. Femern A/S’s May 4, 2026, news release, “First tunnel element of the Fehmarnbelt tunnel to be immersed,” reports that the first concrete tunnel element left the tunnel factory’s work harbour at 9 p.m. that day, beginning its journey to the immersion site off the Danish island of Lolland. The element is 217 meters long, about 712 feet and weighs 73,500 tonnes and Femern’s later release on the successful immersion says standard elements weigh more than 73,500 tonnes, which is more than 81,000 US tons.Five tugboats and the special immersion vessel IVY transported the first tunnel element from the tunnel factory at Rødbyhavn to the immersion point near the future Danish tunnel portal. The release said the transport and immersion would take several days and the element was in place on the seabed by May 7. IVY consists of two immersion pontoons, according to Tunnel Business Magazine and they were released from the element once it was in position and then towed back to the work harbour.Ballast concrete, sealed ends and water chambers that kept the element levelAs the release notes, contractors added a further 4,500 tonnes of ballast concrete so that the element would be heavy enough to sink to the seabed, a figure Railway Gazette International also reports. The element is hollow, sealed at both ends and filled with air during towing, so it floats until the ballast and temporary water chambers let it be lowered in a controlled way. The Fehmarnbelt tunnel will contain four tubes and a service corridor. Two tubes will accommodate a dual two-lane motorway, with two lanes in each direction, while the other two will carry electrified railway tracks, as described by the European Climate, Infrastructure and Environment Executive Agency (CINEA) in its July 30, 2024, overview of the project.Due to the difference in weight between the road tubes and rail tubes, water-filled temporary water chambers in the outer rail tube ensure that the element remains level during submergence. A gravel bedding layer was also laid in the trench in order to ensure that the element was placed in the proper position.About 14 hours of immersion and a hydraulic connection to the Danish portalAccording to Tunnel Business Magazine, immersion began around noon on Wednesday, May 6, 2026 and after about 14 hours the element lay at the bottom of the trench next to the Danish side of the tunnel. Inside the element, the laser measurements showed the correct positioning of the element. According to Railway Pro, the remaining 88 elements will also be put into place in this way over the coming years in a trench that is up to 40 meters underwater. Rail Market reports that after the connection of the element, the rock and gravel are spread on the sides of the element to keep it stable in place in the trench.Construction since June 2020, over €1 billion in EU funding The European Climate, Infrastructure and Environment Executive Agency’s page Linking Germany with Denmark and beyond states that construction began in June 2020 and that the Connecting Europe Facility is co-financing the project with over €1 billion, with completion expected in 2029. The funds are allocated for the construction of the tunnel elements production facility, the portals, the trench and the two transformer stations and for monitoring the environmental impact of the construction. The German Federal Ministry of Transport’s Fehmarnbelt Fixed Link page says that excavation of the 18 km tunnel trench began off the coast of Lolland in July 2021 and it describes the strait as 20 km wide from coast to coast, with the 18 km immersed tunnel and landworks on both coasts forming the link. The Ministry further states that in July 2024, the EU provided another 159.5 million euros under the Connecting Europe Facility funding program for the tunnel. Tunnel Business Magazine reports that when the link is completed, the crossing will take 10 minutes by car and 7 minutes by train.You use AI every day. Now get your AI Quotient. Take the AIQ test.