A vast web of fungal filaments running through subspace and connecting every part of the multiverse, which the USS Discovery travels along by means of its experimental spore drive.
The mycelial network is the discovery of the astromycologists Paul Stamets and Straal, who studied the spores of the fungus Prototaxites stellaviatori. It is introduced in Context Is for Kings, and Discovery's displacement-activated spore drive lets the ship jump instantly along it.
Navigation needs a living mind. A giant tardigrade serves at first, then Stamets himself after he takes on its DNA. The network also leads to the mirror universe, where a Terran reactor is poisoning it in the first season.
In Saints of Imperfection the crew learn that it has its own inhabitants, the jahSepp, and recover Hugh Culber from within it. In the 32nd century the empath Cleveland Booker proves able to pilot the drive as well.