Construction underway on the world’s longest tunnel immersed underwater

If the idea of driving more than 11 miles through the ocean sounds terrifying, you’re not alone. Henrik Vincentsen gets it.

“When you drive through the Fehmarn Belt tunnel, at the deepest point, you will have 40 meters (approximately 131 feet) of water above you,” he said. “So, you are quite deep down.”

Vincentsen is CEO of the Danish state-owned company behind the Fehmarn Belt Link, which will connect Denmark and Germany through the longest immersed tunnel in the world. The project’s price tag is $7.7 billion. Most of the funding comes from Denmark and the European Union.

Developers say the project will fundamentally reshape travel in the EU for the better, cutting the time it takes to get from Hamburg to Copenhagen in half. But German advocates aren’t so sure the benefits outweigh the risks.

Unlike other passages through major waterways — such as the Channel Tunnel between England and France, which travels through solid earth beneath the ocean floor — the Fehmarn Belt will travel along a trench at the bottom of the sea, with water flowing above. The tunnel itself is locked into place with stone and sand covering the top.

Footage of construction crews shows workers casting the structure of the tunnel in concrete. They’re building the tunnel in 79 different pieces, each weighing approximately 150 million pounds. 

Vincentsen said that he understands that the idea is intimidating, but said there’s nothing to fear. The tunnel’s traffic will be split into two separate roadways, making it easier to respond to any issues underwater. It will also include monitors throughout in case of an emergency.

“In terms of safety, it’s probably some of the most-safe areas to actually drive, because it’s well-designed,” he said. 

Danish advocates were initially skeptical of the proposal to connect islands between northern Germany and southern Denmark, especially in early days, when the developers were thinking of building a bridge above the water rather than a tunnel.

“We were against it, because we think it was maybe destroying nature and acting like some sort of barrier for the stream of water in and out of the Baltic Sea,” said Michael Kruse of the Danish Society for Nature and Conservation.

Kruse said he was particularly concerned about the potential for massive ships that travel through the Baltic to hit the structure, causing major environmental problems.

“If you can imagine a Russian supertanker coming through the Baltic Sea and it made contact with the bridge, it could give a lot of problems with the environment amid all that oil,” he said.

The company in charge of the project eventually made concessions, deciding to go with a tunnel. Kruse said environmentalists changed their stance as a result, and now, he looks forward to being able to zip between Denmark and Germany without taking a ferry or finding a lengthy car route. The shorter distance — and the option of traveling via railway — will ultimately be better for the environment, he said, adding that the new connection will foster cultural exchange.

“It’s two cultures meeting in one project,” Kruse said. “It’s almost like you are creating a new country, a new culture — a united culture.”

But some Germans say they see things differently. 

Malte Siegert, with Germany’s oldest and largest environmental group, NABU, said that while a tunnel is a better option than a bridge, the project will primarily benefit the Danish side by creating jobs in one of the country’s poorest areas.

Germany, he argued, doesn’t need the tunnel.

“The area of [Schleswig-Holstein], which is the region on the German side, is very much dependent on tourism and lives on tourism mainly,” he said.

Siegert said he is concerned that rather than helping the tourism industry, the construction and traffic jams caused by the tunnel will destroy the area’s charm for vacationers.

Still, the project’s developers are moving forward, arguing that the economic benefits of a new link between the two countries outweigh any local concerns. They hope to open the tunnel to trains and cars in 2029.