Tunnel elements for the longest immersed tunnel in the world are being cast at the world’s largest concrete element factory in Rødbyhavn, Denmark.

The first 24 metres of what will be an 18km tunnel joining Germany and Denmark have been cast so far, by Femern Link Contractors.

In total, 79 tunnel elements, each 217m x 42m, and nine metres high, will be produced from nine castings or segments, each 24m long. Another 10 elements will be cast in different sizes.

The first element is due to be immersed in the Fehmarnbelt in 2024 in a tunnel trench, which is currently being dredged. More than 90 per cent of the dredging operations have now been completed, the project managers say.

Once set, the elements are pushed out to a dry dock one by one and sealed at both ends with a bulkhead before being towed by tug and lowered onto the seabed.

With millimetre precision, they are fitted together, water is pumped out and a partial vacuum ensures they are locked together.

A layer of protective stone then covers the elements, and over time this will in turn be layered naturally with gravel and sand.

The tunnel will start and finish in Lolland in Denmark and Fehmarn in Germany. As well as containing by road, a railway track will accommodate a 200km/h train that will connect Hamburg with Copenhagen in less than three hours.