The Tunnel is one of the areas in the Town of the Protagonist. It serves as a normal tunnel for people and vehicles to use when entering the mountain or leaving the Town. But it also serves as an entrance to the domain of the Mountain Spirit (as well as its Hand Spirits), as well as an entrance to the afterlife for the Spirits in general.
During the Opening, the Protagonist was walking her dog, Poro, in the road near the tunnel. As it was starting to get dark soon, she decided to return home. As they walked back, a Hand Spirit ran pass behind them. But despite sensing it, they weren't able to see it and thought it was just their imagination. They then continues to walk. Suddenly, a pebble fell from above a wall next to the road, so the girl decided to play a moment of fetch with Poro by using the pebble. However, as Poro was about to retrieve the thrown pebble, a truck came out of nowhere and accidentally run over the dog. It happened so quickly and suddenly, so it knocked the girl unconscious without clear memory of what just happened. When she woke up, all she found was her dog missing, a blood pool, and an empty leash. The girl may have some idea of what had happened, but she chose to deny that Poro was killed and only believed that her dog only went missing due to her age. With no clue what to do, she returned home with the empty leash.
During Chapter 7, upon learning the Black Crayon Note that was left behind by a Mr. Yomawari that her sister (who went missing in Chapter 1) was kidnapped and taken to the other side of the tunnel, the Protagonist went through the tunnel to look for her sister. However, she was stopped by a thick mist blocking the way through the tunnel. Then, she notices her Old Charm is reacting to a small shrine near her. She then interacts with it, causing it to shine brightly and remove the black mist. The girl then proceeded further, only to encounter Meat again. She quickly runs ahead, and ended up lighting another shrine. This causes Meat to attack the shrine. So the Protagonist tries to run, but she encounters a large Hand Spirit. However the large Hand Spirit and Meat ended fighting each other, so the girl was able to escape. Then, she went up the mountain, lighting small shrines and dodging Hand Spirits that were attacking her. She eventually reaches the top to find her sister and a large temple. However, she was attacked by the Mountain God, who teleported them to a different dimension to have the girl killed. The Protagonist was able to defeat the god by lighting up all the small shrines there and was able to return to her world. She then proceeds to pick up Sis and return home with her. However, as she left the tunnel, the Mountain God steals the Protagonist's left eye in exchange for taking her sister back. This causes her to lose consciousness. While she was unconscious, she meets Poro in the Limbo World, right next to the tunnel that acts as the gate to the afterlife. The girl finally bids her dog goodbye before leaving, and Poro crosses to the afterlife. During the event, the Protagonist was brought by her sister to somewhere to have her missing eye treated and then later the two returned Home.
The Tunnel leads to the mountain, which serves as the domain of the Mountain Spirit and its army of Hand Spirits usually at night. It consists of rich vegetation and a mountain that leads to the large abandoned shrine of the Mountain Spirit. It has stairs leading to the large shrine, as well as several smaller shrines (which can activate a light that drives away the Spirits) and Japanese temple gates.
There are five Collectible Items in the Tunnel area.
You can collect this Item from grassy field area inside the Tunnel. You have to stand between two trees to the right of the save point. After that, just move straight south. The red exclamation mark should appear along the way.
You can collect this Item from the entrance of the Tunnel.
You can collect this Item from the in front of the stairs of the shrine of the Mountain Spirit. It is only accessible after you beat the game, in Freeroam Mode.
You can acquire Puzzle Piece 19 from the grass field area, inside the Tunnel, in Freeroam Mode. In the opposite side of the road that has the Jizo Statue, stand in the location below. Then just head straight south until you see a red exclamation mark.
You can also acquire Puzzle Piece 20 from the path with multiple small shrines, on the way to the Final Boss, in Freeroam Mode.