Fehmarnbelt Tunnel between Germany and Denmark, World's Longest Immersed

RailFreight
The 18km tunnel will remove 2 hours from the current 4.5 hour trip between Hamburg to Copenhagen. Consequently, it also removes 2 hours from trips between Hamburg and the Scandinavian countries. Construction should finish by 2029. The 7.1 billion euros project has a repayment period of 36 years.

RailFreight
The tunnel will be built using cut&cover with 89 "elements." It will have four lanes and two tracks.

Screenshot @ 0:46, cropped

https://youtu.be/VnISluP-ZO8?t=254 4:14

If you want to skip the rather obvious economic arguments for the tunnel in this 14:33 video, then start here. From 7:16, I skipped to 8:10 about construction options.

8:41 video

This 3:32 video about the sealant contains some construction scenes.

A 3:18 video of an animation of the construction process.

 This 4:02 animation video includes a narration and is easier to understand.