An energy barrier that encircles the Milky Way galaxy, the Great Barrier caused havoc with the brains of Humans with high extrasensory perception ratings. Described as "a forcefield of some kind," sensors were unable to detect the Barrier, though it was detectable both visually and by its physical interaction with ship deflectors.
The captain of the S.S. Valiant was forced to destroy his ship in 2069 following an encounter with the Barrier that killed three of his crew and affected a fourth that survived. On Stardate 1312.4, the U.S.S. Enterprise NCC-1701 also encountered the Barrier while searching for clues to the Valiant's fate. Nine Enterprise crewmembers were killed, and two more—Lieutenant Commander Gary Mitchell and Doctor Elizabeth Dehner—gained almost godlike abilities before Dehner was killed by Mitchell, and Mitchell was killed by Captain James T. Kirk.