They’re Sinking an 18-Kilometer Tunnel Into the Sea

They built the tunnel on land. Then they floated it into the Baltic Sea and sank it into the seabed. The Fehmarnbelt Tunnel will connect Denmark and Germany with an approximately 18-kilometre immersed tunnel carrying four motorway lanes and two electrified railway tracks. But this is not a tunnel drilled through rock. Its 89 giant concrete elements are being built inside a massive factory, sealed, floated into the sea, lowered into a trench, precisely positioned, connected underwater, and buried beneath the seabed. Each standard element is about 217 metres long and weighs more than 73,000 tonnes. As of late September 2026, four elements had been installed. So how do you sink something this enormous into the sea — and then repeat the process 89 times? In this documentary, we follow the engineering behind the Fehmarnbelt Tunnel, from the factory and floating tunnel elements to the giant immersion vessel, seabed trench, underwater connections, environmental challenges and the delays affecting the project. Sinking one is a method. Sinking 89 is a tunnel. If you enjoy documentaries about impossible engineering, massive infrastructure and the places humans are building that cameras rarely get to see, subscribe to Impossible Earth. #FehmarnbeltTunnel #Tunnel #Engineering #Denmark #Germany #ImpossibleEarth