From Halopedia, the Halo wiki
The Mirror Flats were huge stretches of ice that were flanked by towering ice caves on X50, the moon of Draetheus V. During the Battle of Draetheus V, Merg Vol's Covenant deployed Phantoms to set up supporting artillery along the eastern cliff wall. Spartan Sarah Palmer and a detachment of Marines were sent to infiltrate the Covenant positions and destroy the plasma artillery, while navigating the mazes of cracks. Spartan Palmer used Mirror Flats' harsh terrain to her advantage to defeat Covenant forces. Later in the battle, the UNSC Eminent Domain outmanoeuvred Covenant forces in orbit over the moon and offered some support to the UNSC forces below by deploying Marines to the southern part of Mirror Flats.
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