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...