Maybe you already know, Japan is a chain of islands. There are four major islands, Hokkaido, Honshu or main island, Shikoku and Kyusyu. All the islands are connected with bridges basically. But Honshu and Hokkaido are connected with the tunnel. There is a channel called Tsugaru kaikyo between Honshu and Hokkaido so the tunnel is below the ocean. The tunnel is named Seikan tunnel. There is a Seikan tunnel museum and it has a walking tour to take you a deep and dark Seikan tunnel.
Constructing Seikan tunnel took about 24 years. Since before WWⅡ, people had been thinking about a new way of connecting Honshu with Hokkaido. There had been ships going between them. But after WWⅡ some accidents over it happened and that got people to decide to actually make it happen, building Seikan tunnel. They started investigation and working on it in 1964. You can see the video at the museum showing how tough it was. You may think that constructing below the ocean was too dangerous. Simple mistake could kill people. Not like ground, if water comes into the tunnel, there is no way to get out of there. They say 34 people died in this project. That means how dangerous the work was. Thanks to a lot of work from lots of people, the tunnel is at 240 meter below the sea and 53.85 km long now, which is the longest channel tunnel in the world. It really helps the transportation of stuffs between Honshu and Hokkaido. You can learn other things about tunnels in detail with many displays in the museum too.
Cable car for the tunnel 
Even the tunnel has a train station
The museum provides a walking tour in the tunnel that you can walk inside the tunnel. Riding the cable car and going down the tunnel you will arrive at Seikan tunnel memorial train station in the tunnel, which is a starting point of the tour. There are actually three tunnels below the ocean. Let’s say tunnel A, B, C here. Tunnel A is used for prior investigation. People did a research and construction almost at the same time. They tried to avoid and eliminate risks beforehand by doing it. Tunnel B was used as the main tunnel where trains would run there in the future. Tunnel C was used for putting stuffs for the tunnel such as machines. They first carried stuffs into tunnel C and then move them to tunnel A or B. That means tunnel C is only the way to the ground. In the walking tour, you can walk in Tunnel C with a guide. Guide explains everything and you can learn how it was from displays, figures and signs. Displayed things were actually used during the tunnel building.
There are even bicycles inside. But I guess you already understand why. As I told you, there are three tunnels at the place where Seikan tunnel was built, it was almost impossible to walk everywhere. I wonder how much further would it be from side to side. Also, since the tunnels are below the ocean, you can see water dripping. How dangerous it was to build below the ocean. They did it in sacrifice for their safety. Actually the tunnel management is in financial difficulties now but I hope they try to keep it alive because the tunnel was made on danger, fear and some of their lives.
In Imabetsu city, which is within 30minutes by car from Seikan tunnel museum, there is a park with the best spot to see bullet train coming into and out from the tunnel. The park is located south to the museum. That would mean the tunnel starts here in the park and it is a little inside the prefecture. You may think the tunnel starts at the tip of prefecture but it is not. I suggest you should check train schedule in advance, since bullet train called Hokkaido shinkansen runs here few time. I visited the park only to find I missed, by the way.
