From Halopedia, the Halo wiki
The human empire mined the planet for ores and volatiles. After the human-Forerunner wars, the Didact ordered several monuments built to his fallen Warrior-Servants, one of which was built in the ring system around Sothra Hakkor.
The Warrior-Servant monument orbiting Sothra Hakkor is the setting of the Halo 4 multiplayer map Monolith.
Technically, the rock formation surrounding the planet is a ring system, though it is officially referred to as an asteroid belt.
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