I think Peter Vesterbacka oversimplifies the potential difficulties of building a tunnel from Helsinki to Tallinn. With artificial islands and so.

Here in Hong Kong where I am based at, they have been trying to build a mega crossing with a 18-mile sea bridge and a 4-mile underwater sea tunnel with artificial island entrances. Apart from being delayed, the estimated cost overrun is already at HK$1.5b. I think the complexity of this crossing would be nowhere compared to so-called Finest tunnel.

https://www.thestandard.com.hk/breaking-news/section/4/163520/HK-mega-bridge-project-set-to-exceed-budget-by-more-than-HK%241.5b