From Halopedia, the Halo wiki
In the August 2552, a portion of Noble Team was sent to scout a Covenant "dark zone" at Szurdok Ridge during the Fall of Reach. There they discovered that the area was being used as a staging ground for their forces to invade the planet.
The next day a large battle between the United Nations Space Command and the Covenant took place on the ridge. UNSC forces would eventually make their way to Spire One. There, the CSO-class supercarrier would destroy the UNSC Grafton, sending its wreckage into the ground.
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