The Limit of Questions (Limit of Life in the original Japanese version) is a theory among the scientists in the Eureka Seven series that too many sentient lifeforms in a given space will collapse reality. According to the theory, this will result in a black hole that will tear through space and absorb the entire planet.
The theory was developed by Dr. Greg "Bear' Egan, although his research on it was rejected by many other scientists who denied that the Scub Coral was a sentient being. He further theorized that the Scub Coral had once awakened and reached the Limit of Questions 3,000 years prior to the beginning of the series, but the Scub Coral managed to avoid the total collapse by forcing itself back into a dormant state. However, the effects of the Limit of Questions left a deep mark on the planet, which later came to be known as the Great Wall, which now is the dimension that leads towards the Earth.
In the later part of the series, upon learning that Dewey's plan to destroy the Scub Coral will activate the Limit of Questions, the Gekkostate sets out to stop him and send Renton and Eureka to the Great Wall to find a way to save the planet. However, after he successfully destroys the Scub Command Cluster, the Limit of Questions is activated but reality doesn't fully collapse due to Eureka about to become the next Command Cluster. Eventually, Renton and Eureka awaken the Second Summer of Love, by doing so, they have saved the planet from being destroyed. Afterwards, Nirvash and some of the Scub Coral depart from the world for another in order to prevent the Limit of Questions from happening again.
It is brought up in the final episode of Eureka Seven: AO by Renton as he explains the appearance of the Scub Coral in the alternative universe to his son, Ao, and a few others.