The Griever Hole is an invisible entrance/exit through which the Gladers escape the Maze. It is a small hole located beyond the Cliff.
The Maze Runner
It was discovered by Thomas and Minho while running through the Maze after they followed a Griever who rolled right off the edge of the Cliff. Inside the Griever Hole are some coffin type things which the Grievers rest in, and a computer where Teresa punches the code they discovered from solving the Maps. This reveals the exit to the Maze which allows the Gladers to escape, as well as deactivating the Grievers. It is a small hole that is a few feet square. When the Gladers attempt to escape the Glade, Thomas and the other Gladers jump through the hole and fight the Grievers. They arrive to a room full of Grievers, one of which Thomas spears, releasing a yellowish goo and killing it.
The Death Cure
In the Death Cure, Thomas, Teresa and some other Gladers/Group B return to the Maze since immunes captured by WICKED are being held inside. As the place is coming down and they run slowly as they are escaping with the immune monkeys, some Grievers begin to awaken from their coffins. Teresa tells Thomas how to deactivate them, but one escapes its coffin before it is deactivated, so a difficult battle ensues, where Teresa is almost suffocated by the Griever's moist skin.