We've never seen them directly, yet we know they are there, lurking within dense star clusters or wandering the dust lanes of the galaxy, where they prey on stars, or swallow planets whole. Our Milky Way may harbor millions of these black holes, the ultra dense remnants of dead stars. But now, in the universe far beyond our galaxy, there's evidence of something even more ominous: a breed of black holes that have reached incomprehensible size and destructive power. How big can they get? What's the largest so far detected? Where does an 18 billion solar mass black hole hide?