The tunnel, going 40 meters under the Baltic Sea, will connect Denmark and Germany, reducing travel time between the two countries. It will open in 2029, reports CNN.

The project was planned for more than ten years and only in 2020 the construction of the Fehmarnbelt Tunnel began. This comes months after the completion of the temporary harbor on the Danish side, which will house the factory that will soon build the 89 massive concrete sections that will make up the tunnel.

“The expectation is that the first production line will be ready around the end of the year, or beginning of next year,” said Henrik Vincentsen, CEO of Femern A/S, the state-owned Danish company in charge of the project. “By the beginning of 2024 we have to be ready to immerse the first tunnel element.”

The length of the tunnel will be 18 kilometers, which is one of the largest infrastructure projects in Europe. The budget for its construction is more than 7 billion euros. For comparison: the 50-kilometer tunnel under the English Channel that connected England and France, completed in 1993, cost the equivalent of $13.6 million. Although the Channel Tunnel is longer than the Fehmarnbelt, it was built using a boring machine rather than by submerging pre-built tunnel sections.

The tunnel will be built through the Fehmarn Belt, the strait between the German island of Fehmarn and the Danish island of Lolland. It is designed as an alternative to the current ferry service between Rødby and Puttgarden, which carries millions of passengers each year. The crossing now takes 45 minutes by ferry, while it would take just seven minutes by train and 10 minutes by car.

The tunnel, whose official name is Fehmarnbelt Fixed Link, will also be the longest combined road and rail tunnel in the world. It will consist of two two-lane highways, separated by a service passage, and two electrified railway tracks.