One of the most famous undersea tunnels is the Channel Tunnel, operated by Eurotunnel and connecting the UK to mainland Europe. However, other 'super long' tunnels have been proposed or are currently under development, which will fundamentally change the way both freight and passengers are transported across the globe.

Helsinki to Tallinn Tunnel

The Helsinki to Tallinn rail tunnel is a 92 kilometre tunnel due to begin construction in 2025. With two artificial islands, two rail tunnels and a service tunnel, the tunnel will be located 215 metres below sea level at its deepest point. Freight ferry travel between Helsinki and Tallinn currently takes around 2 1/2 hours. The new tunnel is expected to cut that time to around 30 minutes.

Bohai Strait Crossing Tunnel

The proposed Bohai Strait tunnel, which would connect the Liadong Peninsula to the Shandong Peninsula in China, would be around 123 kilometres long. 90 kilometres of this would be under water. Vehicles (including trucks and vans) would be loaded onto rail carriages and transported at speeds of around 155 mph, shortening the crossing time to around 40 minutes. The ferry which connects both peninsulars currently takes around 8 hours.

Taiwan Strait Crossing Tunnel

Connecting mainland China to Hsinchu in Taiwan, the proposed Taiwan Strait tunnel would be around 150 kilometres long, three times the length of the Channel Tunnel. A politically sensitive project due to the difficult relations between both countries, the tunnel would be 200 metres deep, connecting China and Taiwan with high speed trains.

Fehmarn Belt Immersed Tunnel

The Fehmarn Belt is a strait of water connecting the German island of Fehmarn and the Danish island of Lolland in the western part of the Baltic Sea. Currently linked by ferry between the Puttgarden and Rødby, the proposed Fehmarn Belt immersed tunnel would be the longest ever constructed of this type at around 18 kilometres long. The precast concrete tunnel sections would be 40 metres deep, carrying a double track railway and four vehicle lanes.

Strait of Gibraltar Crossing Tunnel

Proposed as a railway tunnel connecting Tarifa on the coast of Southern Spain and Tangier in North West Morocco, the tunnel would be around 40 kilometres long and 300 metres deep. This would make it one of the deepest tunnels in the world. Unfortunately the depth of the sea and the geological conditions makes construction difficult.

Strait of Sicily Tunnel

Linking Sicily to Tunisia, the 136 kilometre tunnel would enable travel between the two countries in around 90 minutes. Crossing the Sicilian channel at the narrowest point, the 230 metre tunnel would be connected to four artificial islands, two rail tunnels and a central one for service access.

What do you think of super long, undersea tunnels? Are they a viable alternative to the more traditional ferry or bridge?