Engineers are assembling 89 blocks of 73,500 tons like giant Lego at the bottom of the Baltic Sea at a depth of 40 meters to create the world's largest underwater tunnel between Germany and Denmark.

Engineers are assembling 89 blocks of 73,500 tons like giant Lego at the bottom of the Baltic Sea at a depth of 40 meters to create the world’s largest underwater tunnel between Germany and Denmark.

Written by Douglas Avila

Published on 15/04/2026 at 22:00

72 people reacted to this.

The largest underwater tunnel in the world is being built with 89 blocks of 73,500 tons each, assembled like giant Lego pieces on the seabed of the Baltic Sea at a depth of 40 meters with a precision of 15 millimeters

Engineers are constructing the Fehmarnbelt underwater tunnel between Denmark and Germany using a method that seems like science fiction. There are 89 precast concrete elements, each measuring 217 meters in length and weighing 73,500 tons.

These giant pieces are manufactured on land, towed by sea, and positioned on the Baltic seabed at a depth of 40 meters. The precision required for the fit is only 15 millimeters.

The project costs €7.4 billion and will create an 18-kilometer tunnel connecting the islands of Lolland and Fehmarn. It will be the largest immersed tunnel in the world.

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An entire factory was built from scratch on a Danish beach just to manufacture the 89 blocks

To produce the elements, a factory was built from scratch in Rødbyhavn, on the coast of Denmark. The construction of this facility would already represent a megaproject in itself.

Concrete block factory for the Fehmarnbelt tunnel

The tunnel combines a 4-lane highway with a 2-line electrified railway. Separate areas accommodate cars, trucks, and trains, as well as emergency exits.

Mads Schreiner, market director of VisitDenmark, stated: “The new connection will make Denmark more accessible to tourists from Central Europe and is expected to increase the flow of sustainable tourism.”

The project is financed by Denmark through Femern A/S, with reimbursement from tolls over 36 years. Germany contributes €800 million for road connections.

Fehmarnbelt versus Gotthard: two different methods of crossing mountains and seas

Unlike the Gotthard Tunnel in Switzerland (excavated in rock), the Fehmarnbelt uses immersed prefabricated modules. This method allows for faster construction but requires millimeter precision.

Interior of the Fehmarnbelt underwater tunnel with highway

At 18 km, the Fehmarnbelt will surpass the Channel Tunnel in the length of immersed modules. However, the Channel (50 km total) is excavated, not assembled.

According to InfoMoney, the project represents the largest traffic investment in Danish history: 52.6 billion Danish crowns.

Other tunnels under construction in Europe, such as the Lower Thames Crossing, follow a similar trend of crossing bodies of water with submerged infrastructure.

Environmental compensation: 300 hectares of recreational areas and 42 hectares of reconstructed reefs

The environmental licensing required significant compensations. The project will create 300 hectares of new recreational and conservation areas in Rødbyhavn.

Aerial view of the Fehmarnbelt strait between Denmark and Germany

Additionally, 42.5 hectares of reefs have been reconstructed at Sagas Bank as part of the environmental compensation measures.

According to Gazeta do Povo, completion forecasts vary between 2029 and 2031, reflecting possible delays due to the complexity of the work. Assembling 89 modules with a precision of 15 mm on the seabed presents an unprecedented technical challenge.

The official Femern A/S website confirms that the project is progressing in 2026 with the start of module immersion. When completed, the Fehmarnbelt will change the transport map of Europe and shorten the route between Hamburg and Copenhagen by 160 km.

Engineers are assembling 89 blocks of 73,500 tons like giant Lego at the bottom of the Baltic Sea at a depth of 40 meters to create the world’s largest underwater tunnel between Germany and Denmark.

The largest underwater tunnel in the world is being built with 89 blocks of 73,500 tons each, assembled like giant Lego pieces on the seabed of the Baltic Sea at a depth of 40 meters with a precision of 15 millimeters

An entire factory was built from scratch on a Danish beach just to manufacture the 89 blocks

Fehmarnbelt versus Gotthard: two different methods of crossing mountains and seas

Environmental compensation: 300 hectares of recreational areas and 42 hectares of reconstructed reefs

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Engineers are assembling 89 blocks of 73,500 tons like giant Lego at the bottom of the Baltic Sea at a depth of 40 meters to create the world’s largest underwater tunnel between Germany and Denmark.

The largest underwater tunnel in the world is being built with 89 blocks of 73,500 tons each, assembled like giant Lego pieces on the seabed of the Baltic Sea at a depth of 40 meters with a precision of 15 millimeters

An entire factory was built from scratch on a Danish beach just to manufacture the 89 blocks

Fehmarnbelt versus Gotthard: two different methods of crossing mountains and seas

Environmental compensation: 300 hectares of recreational areas and 42 hectares of reconstructed reefs

While an average building in London or New York takes three years to be completed, in China a 26-story tower went up in five days with about 100 workers, and the global modular construction market already moves US$ 95 billion per year.

Australia installs giant concrete structures on the seabed 800 meters from the shore to create an ocean intake for desalination that will draw in saltwater and produce 30 million liters of drinking water per day.

While a traditional school may take several days just to raise walls, in Malawi a 3D-printed school had its structure ready in 18 hours, welcomed children in June, and showed a new solution to the lack of classrooms.

Goodbye traditional washing machine: Panasonic surprises housewives with the NA-G5 featuring four drums, a 13 kg washer and dryer, a 12 kg dryer, independent mini washers, a heat pump, and a 4-in-1 system awarded at the German Design Award.

Material once seen as a poor man’s choice returns as a climate bet, with red clay replacing part of the gray concrete in modern buildings, hospitals, houses, and a cultural center that transforms tradition into a solution against urban heat.

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