The Fehmarnbelt Tunnel, set to become the world's longest immersed tunnel, is currently under construction beneath the Baltic Sea, reaching depths of up to 40 meters. This monumental infrastructure project will connect Denmark and Germany, significantly reducing travel times between the two nations, with an anticipated completion date set for 2029. The initial segment of the tunnel was ceremoniously inaugurated on June 17 by King Frederik X of Denmark, marking a significant milestone four years after the commencement of construction in 2020, which followed over a decade of meticulous planning.

On the Danish coastline, to the east of Rødbyhavn, a state-of-the-art factory has been completed, tasked with the production of the 89 colossal concrete segments that will constitute the tunnel. This facility, completed in the previous year, is deemed the largest and most technologically advanced of its kind by Femern A/S, the Danish state-owned enterprise overseeing the project. Spanning 18 kilometers (11.1 miles), the Fehmarnbelt Tunnel is poised to be one of Europe's most substantial infrastructure undertakings, with an estimated construction cost exceeding 7 billion euros ($7.1 billion).

In comparison, the 50-kilometer (31-mile) Channel Tunnel, which connects England and France and was completed in 1993, had a cost equivalent to £12 billion ($13.6 billion) in today's currency. Notably, the Channel Tunnel was constructed using a tunnel boring machine, a method distinct from the immersion of pre-fabricated sections employed for the Fehmarnbelt Tunnel. The Fehmarnbelt Tunnel will span the Fehmarn Belt, a strait separating the German island of Fehmarn from the Danish island of Lolland. It is designed to serve as an alternative to the current ferry service between Rødby and Puttgarden, which accommodates millions of passengers annually. The journey, which currently takes 45 minutes by ferry, will be reduced to a mere seven minutes by train and 10 minutes by car once the tunnel is operational. Officially known as the Fehmarnbelt Fixed Link, the tunnel will also hold the distinction of being the longest combined road and rail tunnel globally. It will feature two dual-lane highways—divided by a service passageway—and two electrified rail tracks.

"Currently, a train journey from Copenhagen to Hamburg takes approximately four and a half hours," remarked Jens Ole Kaslund, Technical Director at Femern A/S, in 2022. "Upon completion of the tunnel, the same trip will be reduced to two and a half hours. Many people opt to fly between the two cities today, but in the future, taking the train will be the superior choice." The car journey will also be approximately an hour faster, factoring in the time saved by avoiding ferry queues. In addition to the benefits for passenger trains and vehicles, the tunnel is expected to positively impact freight transport, as it establishes a land route between Sweden and Central Europe that is 160 kilometers shorter than the current route.

Presently, traffic from the Scandinavian peninsula to Germany via Denmark can either utilize the Fehmarnbelt ferry or a longer route via bridges connecting the islands of Zealand, Funen, and the Jutland peninsula. The project's origins date back to 2008 when Germany and Denmark signed a treaty to construct the tunnel. It then took over a decade for the necessary legislation to be enacted by both nations and for geotechnical and environmental impact assessments to be conducted.

While the process was relatively smooth in Denmark, in Germany, several entities—including ferry operators, environmental organizations, and local governments—challenged the project's approval, citing concerns over unfair competition, environmental impacts, and noise pollution. In November 2020, a federal court in Germany dismissed these objections. "The ruling came with a set of conditions that we anticipated and were prepared for, particularly regarding environmental monitoring during construction and issues like noise and sediment spill," said Henrik Vincentsen, CEO of Femern A/S, in 2022. "We are committed to ensuring that the environmental impact is minimized as much as possible." Other phases of the project are also underway, including the excavation of the trench that will house the tunnel. Each segment will measure 217 meters in length (about half the length of the world's largest container ship), 42 meters in width, and 9 meters in height, weighing a staggering 73,000 metric tons each—equivalent to the weight of over 13,000 elephants. The factory, which consists of three halls and six production lines, will produce these segments, which will be positioned just beneath the seabed, approximately 40 meters below sea level at the deepest point, using barges and cranes. The positioning of the segments is expected to take around three years, with up to 2,500 people directly involved in the construction efforts. Michael Svane of the Confederation of Danish Industry, one of Denmark's largest business organizations, expressed his belief in 2022 that the tunnel will benefit businesses beyond Denmark's borders. "The Fehmarnbelt tunnel will establish a strategic corridor between Scandinavia and Central Europe. The enhanced railway transfer will encourage more freight to shift from road to rail, supporting a more climate-friendly mode of transportation. We view cross-border connections as a catalyst for growth and job creation, not only locally but also nationally," he stated.

While some environmental groups have voiced concerns about the tunnel's impact on the Fehmarn Belt's porpoise population, Michael Løvendal Kruse of the Danish Society for Nature Conservation believes the project will yield environmental benefits. "As part of the Fehmarnbelt Tunnel project, new natural areas and stone reefs will be created on both the Danish and German sides. Nature requires space, and this project will provide more space for nature," he said. "However, the most significant advantage will be the climate benefit. The faster passage through the Belt will make trains a strong contender against air travel, and cargo transport on electric trains is by far the most environmentally friendly solution."