How Did Supermassive Black Holes Get So, Well, Supermassive?

Supermassive black holes are a bit of a problem. Well, some of them power the most luminous objects in the Universe, spewing out high-energy radiation and matter at close to the speed of light, probably sterilizing all of space for thousands of light-years around them. So if you're too close, yeah, they're more than a bit of a problem. But besides that, though, they're a puzzle. Specifically, where do they come from? Do they grow from normal, stellar-mass black holes that feed off matter glutton...