In the 2250s decade, the Federation astromycologist Paul Stamets discovered the mycelial network. Initially upset by the thought Starfleet might weaponize his research, he later joined Starfleet and became a lieutenant commander. His files were classified.
In 2256, Stamets' colleague Lieutenant Justin Straal oversaw the spore drive installed on the Crossfield-class science vessel USS Glenn. The spore drive was based on the two astromycologists' research on the mycelial network. Klingon Matriarch J'Ula stole the research, but not before Straal managed to falsify the data. (STO - "Age of Discovery" mission: "Secrets")
When J'Ula and her House Mo'Kai fleet attacked Starbase One in the Sol system, the deployment of her mycelial spore-based weapon transported a number of ships to the year 2410. (STO - "Age of Discovery" mission: "Downfall")
During an attack on Andoria, J'Ula deployed her weapon to create a rift between the primary universe and the mycelial network. Klingon dark probes were fired into it and corrupted the lifeforms of the mycelial realm, a habitable area within the network. (STO - "J'Ula's Discovery" mission: "Beneath the Skin")
- The mycelial network underpinned the multiverse.
- The traversable area within it was known as mycelial space. It had directional gravity, i.e. a discernible up and down.