Thorn

The Thorn is an area of space that is highly unstable.

History
The Thorn was the place where the first ship of the Mirror War was lost (NCC-1701-O). Starfleet learned that an unstable area of space made it possible for ships to 'crossover' universes, and planned to 'crossover' to the Mirror Universe in the Thorn. By the end of "Alternate Invasion, Part 2" a portal to the Mirror Universe had been opened and a primary unverse ships started to 'crossover' inside the Thorn.