Hidden gargoyle
From Halopedia, the Halo wiki
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On Backwash go towards one of the extending blue bridges. Find a plant that looks like you can walk on it. Crouch jump onto it and walk up it. Turn around and you will jump forward walk toward the wall and then jump to the other side of the bridge and you will see a gargoyle. You can also jump onto the gargoyle but its not solid so you will go through it.
