Azure Nebula

The Azure Nebula was a class 11 nebula located on the border of the Federation and the Klingon Empire. It was composed of oxygen, argon, theta-xenon, fluorine, and sirillium. 

The nebula was created by the supernova explosion of the star orbited by Erigol, the home world of the Caeliar. That explosion also created a series of subspace tunnels leading to numerous points across and beyond the Milky Way galaxy. 

In 2293, the USS Excelsior encountered a Klingon battle cruiser commanded by Captain Kang during his attempt to rescue Admiral James T. Kirk from Klingon custody. The Excelsior escaped when Captain Hikaru Sulu ignited the nebula's sirillium gas. 

In 2381, the Borg began using the Azure Nebula's subspace tunnel network to invade Federation space. The Starfleet vessels USS Enterprise (NCC-1701-E) and USS Aventine eventually discovered this network, and called for an allied armada to gather in the Azure Nebula against an anticipated Borg invasion. While the two vessels were investigating the network, an armada of over 7000 Borg vessels entered the Alpha Quadrant through the Azure Nebula, destroying the assembled fleet.