€7 billion longest undersea tunnel for 2029?

The Fehmarnbelt Fixed Link, one of Europe’s most ambitious infrastructure projects, is steadily taking shape beneath the Baltic Sea, promising to revolutionise travel between Denmark and Germany.

The 18-kilometre immersed tube tunnel will connect Rødbyhavn on Denmark’s Lolland Island with Puttgarden on Germany’s Fehmarn Island. Unlike bored tunnels such as the Channel Tunnel, this structure is assembled from massive prefabricated concrete elements—each standard section measuring 217 metres long and weighing around 73,000 tonnes, built onshore, floated into position, and sunk into a pre-dredged trench on the seabed.

Once complete, the tunnel will carry a four-lane motorway (two lanes per direction) and two electrified railway tracks, which are hoped will cut crossing times dramatically: just 10 minutes by car and 7 minutes by train, compared to the current 45- to 60-minute ferry ride or lengthy road detours. The project, led by Danish state-owned Femern A/S and costing over €7 billion (financed largely through loans repaid by tolls), forms a key part of Europe’s transport corridors, boosting freight efficiency, reducing emissions via increased rail use, and strengthening economic ties between Scandinavia and Central Europe.

Ambitious 2029 deadline affirmed by both governments

Construction began in 2021, with the seabed trench fully excavated by 2024 and tunnel element production advancing at a huge factory on Lolland. Portals on both shores are progressing, though challenges with a specialised immersion vessel have delayed the critical sinking phase by approximately 18 months. Official sources, including Femern A/S and the German Federal Ministry of Transport, continue to keep the target date for completion in 2029, with ministers from both countries reaffirming commitment in late 2025 and planning a detailed review in early 2026.

While most engineers are sucking through their teeth whenever the scheduled finish date is mentioned, the 2029 date remains the government’s goal. Although, some reports indicate potential slippage due to technical hurdles and separate delays in German rail hinterland connections.

Longest undersea tunnel between 2 countries – there are longer, on land

When finished, the Fehmarnbelt will claim the title of the world’s longest road and rail immersed undersea sea tube tunnel, surpassing current records like the 6.7 km immersed section of China’s Hong Kong–Zhuhai–Macau Bridge. The project is set to establish the standard for a tunnel between Spain and Morocco.

For context, here are some of the world’s longest tunnels overall (as of 2026, focusing on notable transport tunnels; many longer ones are water supply or metro systems):

• Gotthard Base Tunnel (Switzerland) – 57 km (rail) – the longest railway tunnel in the world.
• Seikan Tunnel (Japan) – 53.85 km (rail).
• Channel Tunnel (UK/France) – 50.45 km (rail) – undersea rail tunnel.
• Lærdal Tunnel (Norway) – 24.51 km (road) – currently the world’s longest road tunnel.
• Zhongnanshan Tunnel (China) – 17 km (road).

The Fehmarnbelt Fixed Link stands out not only for its scale but also for its innovative immersed design, marking a new era in sustainable cross-border connectivity. As Europe edges closer to this engineering milestone, the project goes to show the continent’s drive toward greener, faster infrastructure.