A nebula just inside Cardassian space near Minos Korva, in which a Cardassian invasion fleet concealed itself in 2369 until Captain Jellico had it mined.
The McAllister C-5 Nebula is a dense nebula on the Cardassian side of the border, within easy reach of the Federation system of Minos Korva. It appears in the sixth-season Next Generation episode Chain of Command, Part II.
While Picard is held by Gul Madred, Captain Edward Jellico, commanding the Enterprise-D, concludes that the Cardassians mean to seize Minos Korva. La Forge works out from residue on a Cardassian hull that their ships have been waiting inside the nebula, where sensors are unreliable.
Jellico sends a shuttle in to lay mines on the hidden ships. The flying is so difficult that he has to ask Riker, whom he had removed from duty, to pilot it. With the mines in place Jellico forces Gul Lemec to withdraw, give up his fleet's main weapons and hand back Picard.