Given that tunnels are some of the longest and largest infrastructure projects ever made, a lot of preparation, development and negotiation goes into their construction before conveyor belts are installed and earth is moved.
The Channel Tunnel, for example, took eight years between the final proposal and its grand opening, but the concept and many plans and ideas for a tunnel between England and France date back over a century.
Depending on how you define “undersea tunnel”, either it or the Seikan Tunnel in Japan are the longest undersea tunnel in the world, but at present, the longest immersed tunnel ever created is far shorter by comparison.
The Drogden Tunnel, part of the link between Denmark and Sweden that also incorporates the Øresund Bridge, is the largest immersed tunnel ever made, although, by the end of the decade, this may no longer be the case.
Fehmarn Belt Tunnel
Connecting the German island of Fehmarn with the Danish island of Lolland, the Fehmarnbelt Tunnel will, once completed, be the largest immersed tunnel ever made. Once it connects to other pieces of tunnel infrastructure, it will become the largest road and rail tunnel in the world.
Like so many tunnel projects of this type, the concept of a fixed link between Denmark and Germany through their two closest islands has been explored for decades.
Once the Fehmarn Sound Bridge was constructed as Fehmarn was connected to the German Mainland, serious considerations were put into place for a bridge across the Fehmarn Belt, an 11-mile stretch in the Baltic Sea equivalent to the Strait of Dover.
The bridge plan reached a relatively advanced stage, but in 2010 was changed to a tunnel project, which would have similar overall costs with fewer risks of delay or complications, given that the 12-mile bridge would have had one of the longest cable-stayed bridges ever constructed and pushed the concept to its limits.
Instead, an immersed tunnel concept was used, where segments of a tunnel tube are constructed in advance and floated into position before being sunk to the bottom for divers to connect them together.
Whilst far from a new concept, with immersed tunnels having been invented in 1903, this was by far the longest immersed tube tunnel ever constructed, and due to the scope and the ramifications of the economic crisis, it was expected to cost €7.4bn.
This cost complicated the approval process, which required the passing of a Construction Act in the Danish Parliament and the approval of a plan application for the infrastructure work.
Given that the German government had reduced infrastructure spending dramatically, this led to a delay of nearly seven years between the vote in the Danish Parliament and the approval from Germany.
After two years of appeals, in 2020 the project was approved on the condition that it was funded by Denmark (who would earn all toll revenue from it) whilst a €714m tunnel would be constructed in Germany.
This, alongside connections on the Danish side, would lead to the longest rail tunnel in the world and the ability to travel from Copenhagen to Hamburg in just 150 minutes compared to two full days.
Construction is ongoing but is expected to be completed in 2029.
