World's Longest Submerged Tunnel

Fehmarnbelt Tunnel Between Germany and Denmark

The world's longest immersed tunnel is being built between Germany and Denmark.

Once completed, the Fehmarnbelt Tunnel will stretch 18 kilometres up to 40 meters below the Baltic Sea.

After digging a trench in the seabed, workers will build 89 individual segments on land.

The amount of concrete used for the tunnel elements is enormous. Each standard element is 217 meters long, 42 meters wide, and 9 meters high, containing around 73,000 cubic meters of concrete.

In total, over 3.5 million m3 of concrete will be used for the tunnel.

Tugboats tow them out to sea, where they're partially filled with water and then lowered down to the seabed.

The tunnel elements must be placed with extreme precision. The tolerance for positioning each element is less than 30 mm horizontally and 50 mm vertically, showcasing the high engineering accuracy required.

These massive concrete structures are quite buoyant thanks to trapped air.

So, extra concrete is added to help them sink.

Operators connect the segments with hydraulic arms, pulling them together as water gets pumped out.

Pressure builds up, creating a vacuum and forming a watertight seal around rubber gaskets.

Workers can then enter from the previous segment and install a second watertight rubber seal and technical equipment.

Once completed, it'll take trains 7 minutes and cars 10 minutes to pass through.

The tunnel is scheduled to open in 2029. It aims to significantly shorten the journey between Hamburg and Copenhagen for trains to about 2 hours and 30 minutes, reducing it from the current 4 hours and 40 minutes.

The tunnel is engineered to last at least 100 years with minimal maintenance.

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