"What this man speaks of is no vault - it is Limbo, the barrier dimension Kaiju pass through on their way to the graveyard."
- ―Ultraman, The Rise of Ultraman issue 4
Limbo is a dimension that appears in The Rise of Ultraman. It is also referred to as the Kaiju Vault by the USP. Limbo is a dimension where Kaiju pass through before they arrive in the monster graveyard.
The dimension referred to as Limbo by Ultraman was experimented with by the Ultras to allow lesser-evolved species to be free from the threat of Kaiju. However, the Kaiju would always return, and so the technology was abandoned.
Later on, the USP discovered the wreckage of the ship that collided with Dan Moroboshi's. From this they retrieved cloaking technology and a device known as the K-Ray. The K-Ray appeared to disintegrate its targets, and so was put into use by the USP. However, a USP scientist named Doctor Yamamoto had discovered the truth during the 1980s, that the targets of the K-Ray were transported to an unstable alternate dimension. The USP proceeded to fire him and cover up the evidence that the dimension was failing.
Years later, Shin Hayata and Kiki Fuji encountered Dr. Yamamoto, who had since grown to distrust the USP. Dr. Yamamoto fired on Fuji with a K-Ray, sending her to Limbo. Shortly afterwards, Hayata led Dr. Yamamato away from several attacking Kaiju and onto Ureo Ichinotani's jet. There they discussed the existence of Limbo, with Ultraman explaining its history to Hayata. Hayata proceeded to convince Yamamoto to send him to Limbo to rescue Fuji.
Within Limbo, Hayata encountered several Kaiju that had previously thought destroyed, plus several more. Though Hayata and Ultraman rescued Fuji and returned to their home dimension, the already stressed walls of Limbo allowed Bemular to follow through.