The mycelial plane was a realm involved in spore drive.

In 2256, the USS Glenn, one of two ships testing spore drive, crashed into a Hawking radiation firewall upon exiting the mycelial plane during a spore drive test. (DIS: "The Butcher's Knife Cares Not for the Lamb's Cry")

This term was only used once, and might be synonymous or closely related to the Mycelial network, which was described as the "discrete subspace domain" containing the mycelium of Prototaxites stellaviatori and navigated by the spore drive.
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