Area X has been cut off from the rest of the continent for decades. Nature …
From chapter 5: Dissolution "There was something about my mood and its dark glow that eclipsed sense, that made me see this creature, which had indeed been assigned a place in the taxonomy—catalogued, studied, and described—irreducible down to any of that. And if I kept looking, I knew that ultimately I would have to admit I knew less than nothing about myself as well, whether that was a lie or the truth."
— Annihilation by Jeff VanderMeer (Page 175)