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 and intact.

Ruins
Buried below Zone 67 was a vast Forerunner city. First excavated by the ONI, the uncovering was later completed by Onyx Sentinels, which had dug a crater over a hundred kilometers in diameter, on the bottom of which lay the gargantuan Forerunner complex. The city was mostly built of onyx and Chalcedonic quartz, polished to optical flatness, mirroring the sky with superimposed rainbow bands. There were pillars and arches, elevated aqueducts; columned temples with crowns of three-dimensional Forerunner symbols; a forest of sculpted geometries of spheres, cubes, and tori; roads that curved up and twisted into abstract surfaces. The largest columns rose to what had once been ground level. The structures reflected numerous different images as sunlight hit them, forming various shapes and illusions.

One of the massive structures included a dome, three kilometers in diameter. The top of the dome faceted into seven flat surfaces, each with a tall arch leading to the interior, each tens of meters in diameter. On the bottom of this was an ascending staircase, each step a hectare of unbroken polished stone. The interior surfaces of the structure were angled and carved with star charts and Forerunner symbols. Below, seven flat surfaces, each the size of carrier landing decks, were evenly spaced. There was nothing beyond the landing pad save darkness. Around the giant pedestal supporting the landing pad, a helix-shaped staircase led into the depths of the structure. Each stair step was spaced a quarter meter, but they fanned out from the pedestal ten meters. The staircase and the structure were made of a strange, reflective material. Below this was Onyx's vast tunnel network, including the installation's map room.

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.