The Grey Sector is an industrial sector on Babylon 5, comprising 30 levels. These levels are home to atmosphere monitoring, the alpha power station, fabrication furnaces and maintenance facilities. Ever since B5 went online, grey sector has gained something of a reputation for strange activity; strange lights, weird noises, even people vanishing. So much so that Maintenance nicknamed it the "B5 Triangle."
During an incident on Babylon 5, a Soul Hunter along with Captain Elizabeth Lochley, Zack Allan, Michael Garibaldi, and an assault team converged in Grey 4 to find a solution to a station wide threat. A number of souls which had been claimed by the soul hunter were attempting to destroy the station from Grey 4.
Location of environmental support facilities, several storage areas and the cooling plant that feeds into the fusion reactor. In 2260, Boggs used Grey 14A to hold Delenn and Fleet Captain Lennan hostage. 
In 2260, during a search for a missing member of the Babylon 5 maintenance crew, Security Chief Michael Garibaldi discovered a missing level. Until this point, Grey Sector was thought to comprise only 29 accessible levels. Levels 1 through 16 were numbered correctly. The missing level was in between levels 16 and 17. Levels 17 through 29 were actually supposed to be numbered 18 through 30.
Garibaldi deduced the presence of a hidden level by timing the ride between adjacent sectors above or below, then timing the ride between 16 and 17. He then commanded the elevator to stop between the two levels, forced the doors open, and stared into a disarrayed, incomplete corridor. After he exited, the elevator was called and closed behind him with no means of access.
The missing level of Grey Sector was inhabited by a religious cult led by Jeremiah. The people lived there in secrecy and did not believe that escape to the rest of the station was possible. Grey 17 was also inhabited by a dangerous alien known as a Zarg.