Fehmarnbelt Fixed Link: Building the Immersed Tunnel Under the Baltic

Engineers, ship captains and welders are steadily assembling a scheme that could reshape travel in northern Europe, advancing one enormous tunnel unit at a time.

A 19 km shortcut under the Baltic

The Fehmarnbelt Fixed Link will join Rødbyhavn in Denmark with Puttgarden in Germany via an immersed tunnel set on the seabed. When it is finished, motorists and rail passengers will be able to cross the strait in minutes, rather than spending close to an hour using the ferry.

At roughly 18 kilometres in length, the tunnel will rank among the longest immersed road-and-rail tunnels anywhere. Inside, it will accommodate a four-lane motorway and two electrified rail tracks in separate tubes, along with a dedicated service corridor.

The backbone of the entire link is a chain of hollow concrete segments, each as heavy as a small cruise ship.

The units are manufactured on land in a purpose-built factory, then sealed, floated out and hauled by tugboats to the Fehmarnbelt. From there they are lowered with millimetre accuracy into a prepared trench on the seabed.

The arrival of two maritime giants

For months, progress has been held up by one crucial missing component: two vast, specialist vessels built to lift and position the 73,000‑tonne tunnel elements. Without these ships, it would not be possible to place the concrete sections on the seabed with the required precision.

Often likened to “mega-floating cranes” paired with advanced positioning technology, the vessels are bespoke to this job. Each can hold station against wind, waves and currents while guiding an immense concrete block down dozens of metres below the surface.

Each standard tunnel element is roughly 217 metres long, weighs up to 73,000 tonnes, and must be aligned within a few centimetres.

The two ships operate as a synchronised pair: one manages the leading end of an element, while the other controls the trailing end. Their teams work from GPS, sonar and laser references to match the exact coordinates set by engineers onshore.

Why the tunnel needed to “wait” for them

Preparatory work at the Fehmarnbelt has continued in the meantime, including dredging along the route, installing protective layers, and completing the purpose-built element factory in Rødbyhavn. However, the most sensitive phase-lowering and placing the elements-had to be paused until the heavy-lift vessels had finished trials and obtained certification.

Test operations carried out in calmer waters examined ballast systems, winches, cables and safety procedures. If anything were to fail while a 73,000‑tonne block was suspended beneath a ship, the danger to crews, equipment and the marine environment would be substantial.

Only once those checks were signed off could the vessels proceed to the Baltic, where weather opportunities are brief and sea conditions can shift quickly.

How an immersed tunnel is built, step by step

To see what these vessels will do in practice, it is useful to set out the method in distinct stages:

• Excavation: Dredgers remove seabed material to form a trench along the selected alignment, in places reaching up to 16 metres deep.
• Seabed preparation: Gravel and crushed rock are placed to create a firm, level base.
• Element construction: Massive concrete segments are cast at the factory, cured, and outfitted with internal systems.
• Float‑out: The watertight, hollow elements are launched and floated like huge, blunt-ended ships.
• Towing and positioning: Tugboats, together with the two heavy-lift vessels, tow the element and hold it directly above the trench.
• Immersion: Water ballast is introduced in a controlled way and winches lower the segment down to the seabed.
• Connection: Divers and remote equipment connect each new segment to the previous one using gaskets and steel joints.
• Backfilling and protection: Gravel and rock are placed over the tunnel to protect it from anchors and currents.

The two newly arrived ships are central to the last four steps, where accurate control becomes decisive.

Engineering under pressure

Setting a 73,000‑tonne element is as much about control as it is about sheer lifting capacity. Lateral currents in the Baltic can nudge the load, wind forces act on the ships above, and pressure increases as the element descends.

On board, teams monitor banks of displays streaming live read-outs: location, depth, tilt, cable tension, and the remaining gap to the previously installed tunnel section. By adjusting ballast tanks, engineers can shift the element’s centre of gravity while it hangs beneath the hull.

The acceptable margin of error is tiny: alignment must stay within a few centimetres over a length of more than two football pitches.

Once at the seabed, the segment is guided onto neoprene and rubber seals that will become the watertight interface. Hydraulic jacks then draw the new unit towards the one already in place, compressing the seals and securing the connection.

Why size matters for these ships

The ships’ scale is dictated by the mass and shape of the segments. If a vessel were smaller, it would heave and roll more in waves, making fine positioning all but impossible.

A broad hull and multiple lifting points spread forces more evenly, reducing the chance of placing excessive stress on the concrete. The ships are also sufficiently long to balance buoyancy so the combined ship-and-segment arrangement stays stable as ballast changes during immersion.

Transforming travel between Scandinavia and central Europe

The Fehmarnbelt tunnel is frequently presented as a “missing link” between Scandinavia and mainland Europe. At present, most people depend on ferries or take longer routes through mainland Denmark.

Mode	Current typical time	Projected time with tunnel
Car (including ferry)	Approximately 45 minutes on the ferry, plus waiting and loading	Around 10 minutes through the tunnel
Rail (Hamburg–Copenhagen)	About 4.5 hours	Potentially around 2.5–3 hours

Freight movements stand to be altered just as dramatically. Goods trains travelling from Sweden and Norway to the continent will no longer be tied to ferry timetables or vulnerable to weather-related cancellations. Logistics planners anticipate steadier journey times and potentially reduced costs.

Economic and environmental stakes

Authorities in Denmark and Germany frame the tunnel as both a strategic economic route and a climate initiative. By moving long-distance passengers and freight from air and road to electrified rail, emissions could fall on key corridors.

At the same time, environmental organisations have voiced objections linked to construction impacts. The Fehmarnbelt strait supports porpoises, seabirds and sensitive marine habitats. Dredging and underwater noise can disrupt wildlife, and altered currents could affect seabed ecosystems.

Project planners argue that early, extensive mitigation-quiet piling techniques, adapted work schedules, and monitoring-can limit long‑term impact.

Independent scientists will keep monitoring biodiversity in the area for years after opening to assess whether the promised safeguards deliver in practice.

Why immersed tunnels instead of a bridge?

In the early stages, engineers assessed options including a long cable‑stayed or suspension bridge across the Fehmarnbelt. Ultimately, they chose an immersed tunnel for several reasons.

• Weather exposure: the Baltic can be windy and icy; a bridge deck would face more closures.
• Navigation: the tunnel avoids very tall pylons and wide navigation spans for large vessels.
• Visual impact: an underwater link changes the horizon less than a huge bridge structure.
• Rail constraints: gradients for fast trains are easier to manage in a tunnel with controlled inclines.

The trade-off is that immersed tunnels demand intricate marine construction and long-term waterproofing plans. The joints must remain watertight for decades, and maintenance access is more restricted than it would be on a bridge.

Key terms that often confuse people

Project material uses technical terms that can be unclear at first glance. Two that frequently cause confusion are “immersed tunnel” and “segment”.

An immersed tunnel is not drilled through bedrock in the way the Channel Tunnel is. Instead, it is assembled from prefabricated elements that are placed into a dredged trench and then covered over. The structure rests on, or just below, the seabed rather than deep underground.

A tunnel segment here refers to a huge concrete box, already fitted out with internal partitions, ventilation ducting and emergency routes. Much of the electrical and mechanical equipment is installed while the segment is still in the factory, before it ever meets seawater.

Looking ahead: what could this enable next?

The Fehmarnbelt link forms part of a broader European corridor strategy. Transport planners envisage overnight freight services running from Stockholm to Milan without ferry transfers, as well as daytime passenger trains that make rail a stronger rival to short-haul flights.

Techniques refined on this project-especially manoeuvring extremely heavy segments using custom-built vessels-could shape future schemes. Coastal cities preparing for rising sea levels are already exploring whether immersed structures might combine transport connections with flood defences or utility tunnels.

Engineers are also quietly assessing risk cases in the background: collisions with ships, submarine landslides, unexpected seabed settlement, or major loss of power. Each scenario informs redundancy measures, from emergency lighting to cross-passages that allow people to move from one tunnel tube to another.

For drivers who eventually travel under the Baltic in ten uneventful minutes, most of this complexity will never be seen. Underneath, however, a chain of 73,000‑tonne concrete giants-positioned by two equally formidable ships-will carry out their silent task for decades.