The Crescent Nebula was an interstellar cloud in the Alpha Quadrant, located near the Talarian Republic beyond Federation space. The interior of the nebula experienced waves of radiation that eroded shields and disintegrated matter, which led the nebula to be used for trash disposal.
The nebula was also the site of a little-known spatial anomaly that bridged the prime and mirror universes. The mirror universe end of the passageway lay at the outer fringes of the Terran Empire, and was opened periodically by the convergence of two ion storms.
In 2324, San and Dada Noe used the passageway to enter the prime universe, the latter carrying a superweapon known as the Godsend. San planned to use the Godsend to throw the Federation into chaos as preparation for a Terran invasion. Assisted by Fuzz, his mole within Section 31, San seized the weapon and returned to the nebula to await the next opening of the passageway. San's ship was intercepted by a garbage scow crewed by Section 31's Alpha Team. Philippa Georgiou wrested the Godsend from San and decided to detonate it within the passageway, collapsing it permanently. (Star Trek: Section 31)