Coaxial warp drive

The Coaxial warp drive is a propulsion system which folds space in on itself in a much larger scale than a standard warp drive, allowing instantaneous travel along huge distances.

Believed by Starfleet scientists and engineers to be a merely hypothetical technology, it was seen in operation by the crew of the Federation starship USS Voyager in 2374.

Unfortunately, the ship was unstable when it dropped out of warp. In theory, a coaxial warp drive explosion can collapse space within a radius of a billion kilometers. Tom Paris suggested, and implemented, a symmetric warp field to contain any instabilities in a space-folding core. The core stablised shortly after.

The core of the drive was a device that drew in subatomic particles and reconfigured them, thus allowing the space-folding mechanism to work. The device, however, wasn't allowing for instabilities in the particles - Tom suggested using a carburetor-like device, diluting the subatomic energy flow slightly to counter it. With the modification, Steth's ship was able to go to coaxial warp.

Seven of Nine began working on a way to modify a shuttlecraft to go to coaxial warp, but the ship never made it after Tom and the real Steth destroyed the 'carburetor'. (VOY: "Vis à Vis")

Since the drive wasn't seen again, it presumably turned out to be impossible to make a stable version.