If you’ve ever been curious as to how some extreme metal vocalists achieve their vocal sounds, the Internet has the answer.  WikiHow has a very informative page on How to Do Harsh Death Metal Vocals, which contains a bit of commentary by George "Corpsegrinder" Fisher from Cannibal Corpse.  The San Francisco Bay Guardian also has an interesting article on this topic called "Voices From Hell." 

Aside from covering the basics of how to consistently perform guttural vocals without destroying your throat, the articles also mention some interesting information about the different strategies used by vocalists in the industry, such as inhaled vs. exhaled vocals, varying the pitch of your growl, and keeping saliva in the throat to modify and enhance your sound.