Zone 67

Zone 67 was a highly-classified area on the planet Onyx. After the UNSC expeditions uncovered "Alien" artifacts, the Office of Naval Intelligence took over, creating the "Zone" for excavation and extracting of any Forerunner artifacts and technology as an advantage over the Covenant. Zone 67 was the location of an ONI base, supporting a compliment of troops and an Artificial Intelligence. It also housed a Slipspace COM Launcher, one of only three known launchers in the galaxy.

After Camp Currahee was constructed to the south in 2531, a team of Spartan-IIIs from Beta Company went missing in Zone 67 in 2539. ONI cordoned off the area, ensuring that in the future, no more Spartans (or anyone else) would go there. Zone 67's AI helped Camp Currahee through the years, though it was always a "smart" AI and so it was replaced every seven years. At first the AI was Eternal Spring, who was replaced by Deep Winter, who was then replaced by Endless Summer.

In 2552, after John-117 destroyed Alpha Halo, Zone 67 turned active, with huge numbers of Sentinels coming out of the area and destroying the excavation team, eventually attacking Camp Currahee as well. The Sentinels then continued to excavate Zone 67 during The Battle of Onyx. The planet eventually exploded, destroying Zone 67 but, keeping the Forerunner ruins inside it, intact.

Trivia

 * Zone 67 might be an indirect allusion to the modern day Area 51 for its shrouded mystery, namesake, and designation number following the title, though this is just speculation.
 * If you add the numbers six and seven you get thirteen (6+7=13). The thirteen could be because it's considered to be an 'unlucky', and 'evil' number by a vast majority of people, and the Sentinels there were rumored to be ghosts.