A 103km rail tunnel between Finland and Estonia could be open within 22 years according to a feasibility study.
The proposed tunnel would connect the two countries capitals, Helsinki and Tallinn and their airports, and would shorten the travel time between the cities to about 30 minutes, from at least 90 minutes currently by fast ferry.
The joint feasibility study commissioned by FinEst Link, estimated the tunnel would cost between €13 to €20bn (£11.5bn to £17.7bn) with the countries hoping that 40% of the costs would come from the EU funding.
The construction of the tunnel would include the building of two, 400m by 300m artificial islands in the Gulf of Finland with the tunnel 250m below the surface through crystalline bedrock in Finland, and blue clay and ediacara in Estonia.
Tunnelling is planned to start in 2025, starting in Finland towards to the eastern artificial island site, Uppoluloto, using drilling and blasting. The excavation waste would be used to build the artificial islands, which will provide access, ventilation and energy supplies to the tunnelling sites.
FinEst Tunnel plans
Access to the tunnel for vehicles will be provided from Uppoluloto by a spiralling 1.5km long, 9m diameter inclined tunnel.
From the second artificial island to the west, Tallinnamatal, access will be created by two, 215m long vertical shafts to the tunnel below, due to differing geological conditions.
At both access points, construction hubs and safety havens will be constructed for the TBMs to be assembled.
The TBMs will bore two, single track, 10m diameter running tunnels about 70m apart. An 8m diameter maintenance tunnel will run between them and will be drilled slightly ahead to test the geological conditions. Maintenance cross passages will cross all three tunnels at regular intervals.
Six tunnel drives from each island will take place simultaneously, with the estimated cost for the tunnel build only at £7.5bn with rail costs adding a further £2.3bn.
Once completed, the tunnel would operate 40, 200km/h passenger trains, 28, 160km/h car and truck trains and three, 120km/h freight trains each day.
Consultants who have worked on the FinEst Link feasibility study are: Amberg Engineering, Sweco Finland, WSP, Ramboll Finland, Sito, Pöyry, Strafica, Kaupunkitutkimus TA, Inspira and Rebel Group
The governments have said they would evaluate the results in more detail during the spring.
Watch a video about the feasiblty study here