I've done this two ways, and neither is fun. The first tree, only maybe 16" in diameter, i drilled deep holes in with an auger bit and then gradually burned it away, day after day, with gasoline. That was a long and drawn-out process and the ground still sinks slightly in that location each year after I filled it in. When I took down my giant oak (90ft tall by 40-48" diameter) I hired a grinder and he came out with a massive self-propelled tracked machine. It still took him 3-4 hours but the stump was ground down at least 12" below grade. What shocked me was the massive amount of wood chips it produces. From a stump, roughly 4 feet in diameter and no more than 8 inches above grade when he started, it produced six or seven heaping truckloads of chips. The problem is disposing of them, since you can't burn green chips (especially when they're mixed with dirt) and if you leave them on your lawn and just try to mow over them, their decomposition will pull all the nitrogen out of the soil. Here in the city there's no place on my property to just dump them-- lucikly my wife's community garden was willing to accept them in their compost pile. It also took several yards of topsoil to fill in the hole left behind and I'm still trying to get grass to grow in that location. I think the contractor charged me $400, and that seemed fair given the massive size of the stump. If you go this route, make sure you have a wood chip disposal plan.