A very uncommon piercing that goes through your lower jaw.
That's right folks — right though the mandible, where your tongue lies. If you meet someone with this piercing, it'll look like there's a metal ball attached to the jaw, about an inch or two inches behind the chin. Inside the mouth, there'll be another ball on the bottom of the mouth, underneath the tongue. For you biologists, it passes through the submental triangle.
Medically speaking, this isn't one of the best piercings to get. It runs a high risk of infection, and would be very difficult to get rid of an infection. Any infection also has the potential to cause you quite a bit of trouble, since it's nearby a lymph node, a salivary duct and at least two larger blood vessels and could spread easily. Also, avalyn notes that if the piercing migrates toward the jawbone, you can screw up your temporomandibular joint.
A similar-looking piercing is the vertical lowbret, but this is placed outside the jaw (between the jaw and the lower lip).
If you're going to get one of these done, it's imperative that you go to a piercer that knows what zie's doing — do not be the first mandible piercing your piercer has done! Always go to a reputable piercer. And do not pierce yourself, ever.
Props to mirv for anatomy pointers and avalyn for the bit on the TMJ.