Onyx
From Halopedia, the Halo wiki
Unlike most planets encountered by the UNSC, Onyx never had to be terraformed. So named for its huge quaries of Onyx rock, the planet is covered in dense swamp and has many indigenous forms of plant and insect life. It is located in the Zeta Doradus System.
The planet was discovered and first investigated in 2491 with high marks of approval for colonization, however excavations uncovered Forerunner artifacts in 2511. ONI immediately took control of the planet, shutting it off to outsiders and essentially making it classified. In 2531 however, Vice Admiral Margaret O. Parangosky allowed for a UNSC base to be constructed there to be the training ground for the Spartan-IIIs. That base was called Camp Currahee.
In 2552, The Battle of Onyx was fought here.
Shield World
Unknown to the humans until the battle, Onyx is actually a world constructed entirely by the Forerunner, the entire planet was a large ruin, and was literally comprised of trillions of Sentinels. At the core of Onyx lay the enterance to the Forerunner's place of retreat prior to the activation of The Halos, a full dyson sphere known only as "Shield World."
