Why This Tunnel Is Made of 89 Giant Lego Blocks

This isn’t a ship. It’s a 217-metre section of tunnel — and engineers are deliberately sinking it to the bottom of the Baltic Sea. Piece by piece, 89 enormous concrete elements will form the 18-kilometre Fehmarnbelt Tunnel between Denmark and Germany, designed to become the world’s longest immersed tunnel. But building it is nothing like drilling a conventional tunnel. The elements are manufactured on land at Rødbyhavn, sealed so they can float, moved into open water by tugboats, attached to the purpose-built IVY immersion system, ballasted, lowered into a prepared seabed trench and connected underwater with extraordinary precision. And that process now has to be repeated again and again. In this video, we break down: • Why engineers chose an immersed tunnel instead of a tunnel boring machine • How a 73,500-tonne concrete structure can float • How the tunnel elements are manufactured • How IVY lowers them beneath the Baltic • How two enormous tunnel sections are aligned and connected underwater • Why the project needs 89 elements • What the special tunnel elements are designed to do • Why the original schedule has slipped • And why repetition may now be the project's biggest challenge By late June 2026, the first two elements had been successfully immersed, creating more than 500 metres of connected tunnel. The engineering concept works. Now the challenge is turning a handful of extraordinary underwater operations into a repeatable construction system. CHAPTERS: 00:00 A Tunnel That Floats 00:00 Why Build It This Way? 00:00 Digging the Baltic Trench 00:00 Inside the Giant Tunnel Factory 00:00 How Concrete Can Float 00:00 The IVY Immersion System 00:00 Sinking 73,500 Tonnes 00:00 Connecting the Tunnel Underwater 00:00 Why There Are 89 Elements 00:00 The Schedule Problem 00:00 The Real Test: Repetition SOURCES / FURTHER READING: [Add official Femern / Sund & Bælt sources here] If you enjoy megaprojects, construction engineering and the technology behind the world's most ambitious infrastructure, subscribe for more. Would you trust a tunnel built this way beneath the Baltic Sea?