How Denmark Is Building the Impossible 18 km Tunnel Under the Sea

Denmark is building the world's longest immersed tunnel: 79 standard concrete elements, each 217 m long and 73,500 tonnes, plus 10 special ones, floated out and sunk into a trench in the Baltic. And for two years, the sinking waited for one ship. 0:00 A 73,500-tonne block that floats 1:05 Where and why 2:27 Why not a bridge? 4:12 Stage 1: the trench 5:25 Stage 2: a factory on the beach 7:05 Stage 3: making concrete float 8:26 The tunnel that waited for one ship 9:06 Stage 4: sinking and sealing 10:25 The special elements 11:16 Where it stands now Numbers come from the builders' fact sheets and press releases (femern.com, sundogbaelt.dk), the project consultant Arup, Øresundsbron, Germany's Federal Administrative Court and Danish press (Ritzau, Børsen via FemernBusiness) for the 2032 estimate. Status as of October 2026. Photos via Wikimedia Commons, cropped and animated. CC BY 2.0/3.0/4.0 (creativecommons.org/licenses): J Diers; Scandlines; Tony Webster from Portland, Oregon, United States; G och J; Thomas Dahlstrøm Nielsen; Johan Wessman / News Oresund; Fabian Horst; Willy Pragher; Lukas Beck; News Oresund; Freddy Fehmarn. CC0: Olaf Kraak; Koen Suyk / Anefo. Public domain: NASA; United Kingdom Hydrographic Office. 3D: Blueprint Explained, simplified, not to scale. Music: original, generated in code by Blueprint Explained. Would you drive 18 km under the sea? #Fehmarnbelt #engineering #megaprojects