Comment The sections float and are towed into place (Score 2) 102
Click on the large Play button in the middle at the top of this page: https://femern.com/the-tunnel/...
Or use this link, this is the video they embed: https://vimeo.com/1106084333?f...
It shows the whole process.
First, they dredge a 12 meter deep tunnel trench on the seabed.
Meanwhile, the sections are build in a factory on shore. Sections of 24 m pieces, 9 of them, adding up to 217 (after connecting them). 73000 tons total.
That long section is assembled into a drydock in front of the factory. Both ends are covered and made waterproof. Then the drydock is flooded. The long section is towed into the sea and placed above target location. Using ballast tanks inside the section it is lowered into place with few centimeters of tolerance. The new piece is pulled the last few centimeters using hydraulics, and with the rubber between sections this produces a watertight seal.
The lowered section is then covered in several layers of dirt and finally stones, creating a new seabed over the tunnel.