Boundaries

Boundaries  境界 (Kyōkai) are the seas that connect the Near Shore with the Far Shore, and they can be accessed in various ways.

They were introduced in Chapter 4, when Kou described his older brother telling him about these boundaries that connect both shores.

There is a stretch of water running between deserted buildings and telephone poles, and the sun can be seen descending in the horizon. Kou also mentions how forgotten objects such as used dolls, broken television boxes, and other vintage articles are discarded and have drifted to piles.

When a sacrifice is offered, all boundaries leading to the Far Shore will be closed. It will force all supernatural inhabitants to be moved to the Far Shore, whether or not they have a rumor once their existence is forgotten by the living. This is portrayed by the shattering of their bodies into glass shards.

Locations
There are several spaces in Kamome Gakuen where one can access the boundaries: Most of the School Wonders' boundaries contain water. At the end of Chapter 40, Tsukasa also reveals Shijima hiding under the tablecloth as water can be seen under her feet in the broadcasting club's room.
 * No.2 The Misaki Stairs
 * No.3 Hell of Mirrors
 * No.4 Shijima-san of the Art Room
 * No.5 The 4PM Bookstacks
 * No.6 God of Death

Near Shore (Human World)
The Near Shore (alternatively known as the human world) is a place where the living exists. Only those from the Far Shore can exist on the Near Shore through rumors or powers. Teru admits there are one-way routes to the boundary closest to the Far Shore, such as the run-down building he came out from Chapter 69. However, more formally, there is a gateway between both worlds hidden in an open cave. There are salactites hanging from the wall, and tassels weaving through the cave walls. In order to activate it, Teru bit his finger to release a bit of blood. After drawing a vertical line on one of the walls, it opened a gaping tunnel of water that led to one of Kamome Gakuen's hallways. Guided by the light at the end, one must not look behind them while treking through or otherwise, they won't be able to go back.

Far Shore
The Far Shore is a place known to be the world of the dead. The closest boundary to the Far Shore is No.6's boundary. There is an open pit in the middle of the interwoven Japanese-styled building, rumored to be the entrance to the Far Shore. When a living person enters the Far Shore, their body will deteriorate until it will only leave their soul behind.

However, it was also mentioned that Sumire's village was relatively near a cliff that led to the Far Shore. There are spikes submerged in the water below. Without a sacrifice, monstrous creatures would spawn from the depths of the water and wreck havoc, which led the villagers to sacrifice a chosen maiden of the Akane family.

Creatures
There are many creatures that make their home in these boundaries. They are notable for their otherworldly appearance or decomposing nature. In the presence of humans or supernaturals, malicious creatures will still attack all the same. They're only defeated through an exorcist's weapon or a protection charm.

Aquatic
When Teru, Akane, and Aoi made their escapade by traveling through the boundary, they witnessed different creatures, both hostile and docile. They usually resemble aquatic or amphibian-like animals, such as lizards, turtles, and fish. Some of them usually make their homes in abandoned buildings or mossy caves.

No. 6's Boundary
In the pit, to which Aoi quoted to be the boundary's garbage disposal, is a home to a festering swarm of bug-like creatures, ranging from humongous centipedes with multiple eyes, cyclop moths, and black beetles. They're shown to be a bit sentient, considering Aoi's control over them and unexpectedly stabbing Akane when Aoi distracted him.

Star Festival
These creatures are civil and sentient, as shown by their docile nature and trading of currency (with mermaid scales ranking a high price). They host a wide variety of game and food stands, which closely resemble the humans' Tanabata festival when Nene noticed she was in a different timeline. They're personified to be more like humans, with the exception of their otherworldly appearances matching oni or wind-up toys. They also utilize the rumor of hanging a tag on the festival's bamboo tree for a wish.