Stellar mass black holes, the ones formed by collapsing stars, are thought to be the most abundant. Occasionally they get trapped in each other's gravitational pull and merge in a process of spectacular cosmic violence. It was through such a "binary merger," about a billion and a half light years away, that humanity first managed to tune in on gravitational waves, the elastic rippling of spacetime that radiates away the energy released by the merger. Black holes thousands of times more massive could have formed in the early universe, when exotic objects called quasi-stars were thought to abound, powered not by nuclear reactions but the infall of hot gas towards a rapidly growing central black hole seed. More massive still are supermassive black holes, with masses up to billions of times that of the sun, thought to be the gravitational anchor at the center of larger galaxies like ours. How these form remains something of a mystery.