An ambitious underwater road and rail tunnel is currently under construction in Europe that will link Germany and Denmark. Named the Fehmarnbelt tunnel, it will cross an 18-km (roughly 11.2-mile) stretch of the Baltic Sea.
The Fehmarnbelt tunnel (aka Fehmarn Belt fixed link) is being created by Femern A/S, Rambøll, Arup and TEC, and is described by the team as the world's longest immersed tunnel (i.e. a tunnel built elsewhere and then sunk into place) and the world's deepest immersed tunnel with road and rail traffic. The immersed part is important as there are longer undersea tunnels, such as the Channel Tunnel, for example, but it's still an extraordinary undertaking.
Once complete, it will connect Rødbyhavn in Denmark with Fehmarn in Germany, and will be the shortest route between Scandinavia and the rest of Europe. Travelers will either take a seven-minute train journey or a 10-minute car drive to cross the Fehmarn Belt, replacing a trip that would otherwise have taken 45 minutes by ferry.
To put its size into perspective, the construction of the tunnel will require 360,000 tonnes (396,832 short tons) of rebar, which is the equivalent of almost 50 times the weight of the Eiffel Tower's metal structure. The build site at the Danish side is the size of 373 soccer pitches. Up to 70 vessels were involved in the dredging of the tunnel trench, which is also 18 km (11.2 miles). In total, around 12 million cubic meters (almost 424 million cubic ft) of soil has been dredged from the seabed.
The immersed tunnel itself consists of 79 standard and 10 special sections. Each of the standard sections weighs around 73,000 tonnes (80,468 short tons) and has a length of 217 m (711 ft), with a width of 42 m (138 ft) and a height of 10 m (33 ft). The smaller special sections are under half the length but are a little wider and higher. The tunnel sections are being cast on land and then floated into position using a barge, before finally being submerged and sealed on the sea bed at a depth of up to 40 m (131 ft).
The Fehmarnbelt tunnel's budget so far is around DKK8.4 billion (roughly US$1.2 billion). Denmark's King Frederik X recently inaugurated the first section of the tunnel during a special ceremony and it will be sunk in the coming weeks.
The Fehmarnbelt tunnel is expected to be completed in 2029 and to be in operation for a minimum of 120 years. Check out this video explainer for more information.
Sources: Fehmarnbelt tunnel, Arup