I realize most of you probably agree that Cthulhu is unstoppable, unless you've drunk the "he was beaten by a steamship" flavor-ade. But here are the reasons that he is Earth's poison pill. First, he's not all alone. He's no Godzilla nor even Galactus. He's the ruler of an entire nation of starspawn, each one of them huge, potent, and mind destroying. Some are Great Old Ones on their own (notably Ghatanothoa, Zoth-Ommog, Coinchenn, and Cthylla, among others). We don't know how many starspawn lurk in R'lyeh with him, but it could be millions. Imagine millions of Godzillas rising from the deep. Second, he knows what we're up to. He has been communicating with other starspawn and aliens and cultists constantly. He knows everything going on on Earth right now, and his agents have deeply infiltrated us. He can READ OUR MINDS so we have no secrets anyway. He's super-intelligent. He is smarter than the smartest human who ever lived and he has lived for billions of years. He knows it all. He knows all science thoroughly. Our greatest accomplishments are all things he knew a billion years ago. Nukes? Super-colliders? Strangelets? He has a handle on it all. His alien power lets him blast our minds. Create structures that twist space-time. Open other dimensions. The H-Bomb is a toy. An obsolete toy for him. He's mobile. He can fly and he can plunge through space-time using wormhole travel. He can open portals to anywhere and surge through. Picture him blotting out the sun. Or appearing through a wormhole in the middle of our mightiest fortress. Millions of gigantic, super-intelligent, flying monsters with dimensional powers. Our own defenses are sabotaged by cultists. We go mad just seeing what he has. Anyway. I think about this kind of thing all the time.