The argument for a fixed link is old and simple. Since 1963 Fehmarn has been joined to the German mainland by a bridge, and everything north of it has depended on ferries. By the late 1990s feasibility studies pointed to a cable-stayed bridge carrying four motorway lanes and two electrified tracks - roughly 20 kilometres of it, in three spans, on pylons about 280 metres tall with 65 metres of clearance for shipping. Dissing+Weitling handled the look of it, COWI and Obermeyer the engineering. That scheme was the favourite for years and was expected to open in 2018. Then, in December 2010, Femern A/S - the Danish state company running the project - concluded that an immersed tunnel carried fewer construction risks for about the same money, and a large majority of the Danish parliament backed the change in January 2011. A cable-stayed span at that scale would have been pushing the technology; a tunnel would not.

Everything now turns on a plant at Rødbyhavn on the Danish coast, built from 2021 and completed in August 2023, which exists to cast tunnel segments and nothing else. The first element was finished that December. There will be 89 in total - 79 standard sections of 217 metres and ten shorter special elements - consuming something like 3.2 million cubic metres of concrete. Each finished box is sealed, floated, towed out and sunk onto a prepared bed. Preparing that bed was its own problem: the deepest part of the Fehmarn Belt trench is around 35 metres and the tunnel elements are about 10 metres tall, so the dredgers needed to reach past 45 metres, well beyond the roughly 25-metre limit of conventional equipment. Grab dredgers and trailing suction hopper dredgers did the deep middle. By May 2022 half the trench was cut; by January 2023, seventy percent.

The cross-section is the elegant part. Each element is about 41 metres wide and 10 metres high, and inside it runs four separate passageways plus a service corridor: two road tubes, each 11 metres wide with two lanes and a breakdown lane, and two rail tubes, each 6 metres wide. The 3-metre service passage sits between them. Road runs on the western side, rail on the eastern, which matches how the motorway and railway already lie on both coasts, so nothing has to cross over itself at either portal. That is the whole point of the exercise. Drivers will cross the belt in about 10 minutes and rail passengers in seven, against forty-five minutes on the water plus whatever the queue and the loading take.

Approved in 2015 with a construction budget of 52.6 billion Danish kroner - around 8.2 billion US dollars - this is the largest infrastructure investment in Danish history, to be repaid through tolls, with Germany funding the connections on its side. Getting to a spade in the ground took a decade of law. German authorities approved the project on 28 December 2018 and Femern A/S received plan approval on 6 February 2019, but objectors took it to the Federal Administrative Court. In November 2020 the court ruled construction could proceed while requiring that certain reefs be accounted for; the conservation group NABU called the decision a dark day for the marine environment. A construction freeze near protected reefs followed in January 2022, and the last complaints were dismissed that December. A parallel criticism has never gone away: the route was conceived when a link east to Rostock was politically unthinkable, and some argue a Gedser-Rostock crossing 50 kilometres away would serve a reunified Europe better.

Work formally began on the Danish side on 1 January 2021 and in Germany that November. The official target is 2031, and it is worth noting how that number arrived: the crossing was once meant to open in 2018, then 2021, then 2028, then 2029. German reporting in 2025 suggested the end of 2032 was more realistic. Meanwhile the surrounding works continue - a new 1.7-kilometre Fehmarn Sound tunnel, costed at roughly 714 million euros, is meant to relieve the 1963 bridge, which will be kept for pedestrians, cyclists and slow traffic rather than demolished. At Rødbyhavn a 24-metre viewing platform opened at the start of 2025 so that people can watch. Eighty-eight elements to go, one at a time, each one sunk and joined to the last.