How was the world's longest submerged tube tunnel built?

The Fehmarnbelt tunnel connecting Germany and Denmark is about 18 km long, made up of 89 sections and is expected to be completed in 2029 .

The Fehmarnbelt Tunnel (Fehmarn Belt Hybrid Road) built by Femern A/S, Rambøll, Arup and TEC is the world's longest and deepest submerged tube tunnel with a combined rail and road network. Upon completion, the tunnel will connect the town of Rødbyhavn in Denmark with the island of Fehmarn in Germany and will be the shortest route between Scandinavia and the rest of Europe. Passengers will need just 7 minutes by train or 10 minutes by car to cross the Fehmarn Tunnel, compared to 45 minutes by ferry. The project will consist of two dual carriageways, separated by a service corridor and two tram tracks.