Total immersion: the world's longest immersed tunnel

Denmark is no stranger to challenging infrastructure projects. Over the years – primarily in an effort to improve the transport links between the country’s mainland and its largest islands – it has built a host of record-breaking bridges and tunnels that have cemented its reputation as a global centre of structural engineering excellence.

But now, its civil engineers are poised to begin a construction project that could be the country’s most challenging yet: an 18km-long sub-sea tunnel that will link the German island of Fehmarn with the Danish island of Lolland. Its proponents claim that it will fundamentally transform transport in the region, replacing around two million ferry journeys annually, and slashing rail travel time between Copenhagen and Hamburg by more than 25 per cent.

Dubbed the Fehmarn Belt Fixed Link (after the Baltic strait separating the two islands), the new link will be the longest immersed tunnel ever built and will be five times longer than the current record-holder, the Øresund tunnel, which is also in Denmark. Constructed from vast pre-fabricated concrete sections that will be installed in a trench on the seabed up to 35m beneath the surface, the tunnel will comprise twin railway lines, four motorway lanes and a separate emergency tube.