Inversion drive

The Inversion drive was an alternate development of faster-then-warp travel that was being considered by the Federation and was field tested on the USS Enterprise under the command of Captain James T. Kirk. The technology was created by the Hamalki scientist, K't'lk. While it was capable of intergalactic travel, the drive caused a great deal of damage to the fabric of the universe making it dangerous for use and thus research as well as development of this technology was discontinued.

(TOS novel: The Wounded Sky)