Lunar Complex G/T/39

Lunar Complex G/T/39 is a collection of Lunar bases inside the Letrone crater, surrounded by the Atlas mountains. The complex is made up of Luna Garrision F/A/72, the UNSC Flight Instruction Academy, and eight "nodes", outposts which are stocked with troops and serve as sentry posts. The Complex's main defense is the Atlas mountain range, which completelu surrounds it, except at Blackmoon pass. One Node, Node Alpha, is placed outside the pass. Throughout the mountains, 1,000 50mm hidden flak guns dot the cliffs, making aerial attacks very hard. This means that only ground assaults are practical, which is an advantage. In 2552, the complex was stormed during the defense of Luna. 
The Covenant landed outside the mountain range, regrouped on the ground and annihilated Node Alpha, then swept through the pass, destroying Node Bravo. The Covenant force then split, destroying each Node one by one and encircling the humans. Survivors from each assault managed to make it back to the main complex, and the humans held the line there and beat back the Covenant. The assault faltered, allowing the 5 squadrons from the base to launch and destroy the Covenant picket that had deployed the troops. This however, is speculation, based on comm chatter; none of it has been confirmed, and is kept under tight secrecy by ONI.