How about the Hundred Mile Forest? It's bigger than that, as you say, but it's called that because you can travel through it a hundred miles and never see the sky through the leaves or bare dirt/bedrock through the undergrowth. It's so huge and old and dense there are probably fires there the size of Texas (from time to time) that no one knows about except some druids whom no one in the civilized lands knows about either.
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Or, if its' mostly in some general direction on your main continent, just have everybody call it the Great Western Forest or whatever, and everybody knows that's where orcs come out of to raid the civilized lands...