If making a tunnel to cross a mountain is complicated, doing it underwater is much more difficult, and in general they are done using machines called tunnel boring machines that drill the surface several meters below the seabed.
This is how, among others, the tunnel that connects France with England through the English Channel was built, which has a length of 50 km, and through which only trains circulate to avoid the possible complications of car accidents.
Another tunnel is currently being built to link Germany and Holland. It is the so-called Fehmarnbelt Fixed Link that will have a length of 18 km under the sea, but it will not be done by means of tunnel boring machines, but by means of a cement structure buried at the bottom of the sea. And inside that structure, trains and cars will be able to circulate in both directions
The great novelty is that this structure will be manufactured on land in a modular way, and then the modules will be buried underwater, joining one another. In this way, costs are greatly reduced, although the work has an impressive scope.
An explanatory video can be seen at: https://youtu.be/zf7C5DiEVpA
This project is a good example of how much technology has advanced. I am convinced that this tunnel could not have been built a few years ago, and even today its construction will take more than 6 years.
Currently this route is done by ferry that takes 45 minutes to complete, in addition to the inconvenience of boarding and disembarking. With this tunnel the trip would take between 7 and 10 minutes. The advantage is very clear.
That said, it is a very nice and interesting project. I strongly advise watching the video which gives us an idea of its magnitude.
All that remains is to hope that the project is carried out successfully and with a minimum of accidents. I think it is a great technological advance.
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