The tunnel is 53 kilometres long under the ocean which is 23 kilometres wide at this point. The tunnel is 240 metres below the surface of the Tsugaru Straits and the water is 140 metres deep at the deepest point above the tunnel. It takes the train about 24 minutes to traverse this tunnel and the ride is very smooth. It feels like a jet floating in the sky. Currently there are no "shinkansen" or bullet trains on Hokkaido, but there are plans to have the shinkansen replace the current express trains.
There are countless tunnels in Japan, mostly beneath mountains and many of them were originally built using manual labourers, resulting in significant losses of life.
The first photograph is Honshu, the chart was on each train seat and the other two photographs are of Hokkaido. Notice the very simple style of the houses.