Characters have described the Underworld as being impossible or near impossible to escape (for humans, according to Kofuku; as well as for gods and shinki, according to Yato and Nora) due to Izanami's desire for company. For example, if one eats or drinks anything within the Underworld, they will no longer be able to leave. For this reason, it is said that gods are usually present to protect the area around the entrance; however, on Kamuhakari, their protection is absent.
The entrance to the Underworld is in located in a forest, and appears as a cavernous darkness between two large boulders. The entrance lies beyond two stone pillars held together by enclosing rope, which resembles the real-world location of the entrance in Shimane prefecture. In the anime, the entrance is shown to emit a purple hue. In the manga, Nora was able to tell that the entrance was recently opened by Ebisu because of a swirling pattern within the darkness; this may imply that when it is not "open," this location could be a normal cave on the Near Shore.
The inside of the Underworld also resembles a deep, craggy cave whose terrain varies from place to place. Some areas are more open with a high ceiling, while others are more narrow and cramped. Some areas have standing or running water, in the form of rivers, puddles, or ponds, while other areas are completely dry. According to Yato, all of the water in the Underworld produces a rancid stench. This water seems to allow for some plant life, as roots were shown to be growing along stone walls.
There are many natural rock structures within the Underworld. However, it also seems to contain many stone structures which appear man-made, such a tower, tunnel, and set of stairs. These were likely created by Izanami, who, as the reigning deity of the Underworld, is able to manipulate its layout. When Yato and Ebisu encounter her, she is in a room she had seemingly created for herself. (In the anime, it appears almost instantaneously; in the manga, Yato and Ebisu are transported to her by her Yomi entities.)
This room is structured so that most of it is filled with water. The sides of the room are lined with brick archways, which hold lanterns to illuminate the space. A square pathway extends from the front of the room, creating a small pool or moat directly in front of Izanami's "throne". This area contains a painted paper screen, a bamboo mat, and a flat cushion for her to sit on. She sits with an iron cauldron on one side, and a box on the other. She uses this box for burning incense and her pipe.
The concept of the Underworld was first introduced in reference to Kofuku and her shinki Daikoku (Kokki), who has the ability to open vents. A vent is a direct connection between the Near Shore and the Underworld, allowing phantoms, shinki, humans, or gods to pass through. However, since Izanami is able to control the Underworld, she can seal off the exits, including vents, at will.
The Underworld was first directly mentioned in chapter 28 of the manga (although it was not shown visually until chapter 29); in the anime, the Underworld was introduced in Aragoto episode 8 and shown in Aragoto episode 9. In both cases, this was following Yato's disappearance to complete jobs with Nora for Father. When Yato tried to escape and return to Hiyori before she would forget about him, he was told that he would be free from them after completing one more mission, which would involve journeying to the Underworld to rescue Ebisu. In the manga, Father himself told him this; in the anime adaptation, Nora did.
In the next chapter/episode, Nora and Yato arrive at the entrance to the Underworld. Nora provides Yato a protective shield against phantoms as they enter. They eventually meet Ebisu, who had entered the Underworld in search of the Locution Brush, and successfully return him to the Near Shore. Bishamon later ventures into the Underworld as well in order to rescue Yato. She and Yato are then removed from the Underworld through Hiyori's Soul Call.
Some time long before the beginning of the series, Father had also ventured into the Underworld. He obtained the first Locution Brush and then was removed using a Soul Call, presumably by the unnamed freckled woman who Izanami took the appearance of.