The Great Barrier
The Great Barrier is a spectacular energy field located at the center of the Galaxy that was thought to be impenetrable by any starship.
This article comes from Star Trek Magazine, v. 1, i. 12.
While the galactic barrier is at the edge of the Milky Way galaxy, the Great Barrier surrounds the center of the Galaxy. It crackles with energy, and a starship's sensors will not function in or near it. Until 2287 there were no records of a starship having successfully passed though the Great Barrier; and because no probe sent into the phenomenon had ever returned, it was believed that any vessel attempting to breach the Great Barrier would be destroyed.
However, these reports were proves to be inaccurate by Sybok, the half brother of Ambassador Spock. He believed that the mythical Vulcan planet of Sha Ka Ree (the equivalent of the human heaven, or Eden) lay beyond the Great Barrier at the center of the Galaxy.
In order to prove his beliefs, Sybok hijacked the U.S.S. Enterprise NCC-1701-A and its crew, and set course for the Great Barrier. Despite all previous evidence, Sybok was convinced they would survive then transit through the Great Barrier because 'God' had told him so in a vision.
As the Enterprise approached the Barrier, a giant blue-green field of crackling energy filled the viewscreen. Helm Officer Sulu voiced his doubts about navigating the phenomenon, but Sybok insisted that they continue. The starship ploughed directly into the Barrier and through an incredible conduit of light and color before emerging into a serene area of space. Sybok thus proved that the danger posed by the Great Barrier was mostly illusory, and that a vessel could pass through it without sustaining serious damage.
Once beyond the Great Barrier, they discovered a blue planet with a dry desert surface and a resident life form. At first Sybok believed that he had found Sha Ka Ree and God. However, it soon became apparent that this was not God, but an extremely powerful being who had been imprisoned by the Great Barrier for thousands of years. This malevolent being had used Sybok to lure a starship to the planet so it could escape. However, it was destroyed by the combined crews of the U.S.S. Enterprise and a Klingon Bird-of-Prey after it attacked Captain Kirk and his landing party.