Incredible new £6bn underwater tunnel will be world’s longest and connect 2 countries

An £6bn underwater mega-tunnel linking Denmark and Germany has taken a major step forward after engineers successfully immersed the first section of what will become the world’s longest submerged structure. The Fehmarn Belt Tunnel, stretching 18 kilometres beneath the Baltic Sea, is set to eclipse all existing immersed tunnels once completed – more than tripling the length of the current record-holder, San Francisco’s 5.8-kilometre Transbay Tube.

The milestone marks a key phase in one of Europe’s most ambitious infrastructure projects, designed to physically link continental Europe with Scandinavia and reshape transport flows across the region. Engineers have now lowered the first of dozens of massive prefabricated tunnel elements into a dredged trench on the Danish side of the Fehmarn Belt.

Each section is built on land, floated out to sea and then carefully sunk into position on the seabed in a highly choreographed operation involving multiple European specialist teams.

The work is being delivered through the RAT Joint Venture – a consortium of engineering firms including Ramboll, Arup and TEC – working alongside project client Sund & Bælt.

The group has been advising on technical design and delivery since 2008.

Once complete, the Fehmarn Belt Tunnel will carry both road and rail traffic. It is designed as a dual highway and double-track railway, allowing lorries and cars to pass beneath the Baltic alongside high-speed passenger and freight trains.

Officials say the fixed link will dramatically cut journey times between Germany and Denmark while offering a weather-independent alternative to ferry crossings.

It is also intended to shift freight away from roads and onto rail, with the aim of reducing emissions across a key European transport corridor.

At its deepest point, the trench lies around 45 metres below sea level, with the railway running at roughly 40 metres below the surface.

Once finished, the 18-kilometre structure will become the longest immersed tunnel in the world and a central artery in Europe’s transport network – a project defined as much by engineering complexity as by its scale.