Khecharimudra
Khecharimudra is a hatha yoga practice which involves surgically lengthening the tongue (and by some accounts splitting it as well). The tongue is then flipped back inside the nasal cavity for use in breath control related yoga. Svatmarama describes this practice in the still-popular 15th century text The Hatha Yoga Pradipika. The following translation (offsite link) is from Brian Dana Akers. Notes in italics have been added.
Turn the tongue backward and insert it into the skull cavity. Direct the gaze to the middle of the brows. This is Khecharimudra. Gradually elongate the tongue by cutting, shaking, and stretching it until it touches the middle of the brows. Then Khechari is accomplished. Take a smooth, stainless knife — very sharp, like asnuhi leaf {milk hedge} — and cut a hair's breadth with it. Then rub it with a powder of rock salt {naturally occurring salt} and yellow myrobalan {a powder made of an astringent fruit similar to a prune containing tannin; this powder stops bleeding by constricting blood vessels}. When the seventh day arrives, cut a hair's breadth again. Practice regularly in this manner for six months. The tie of blood vessel at the root of the tongue disappears after six months. Turn the tongue backward and insert it into the path of the three. This is Khecharimudra. It is also called Vyomachakra. The yogi who holds the tongue upward for even half a second is saved from poison, disease, death, old age, and so on. He who knows Khecharimudra is without disease, death, lethargy, sleep, hunger, thirst, and fainting. He who knows Khecharimudra is not tormented by disease, not smeared with Karma, not troubled by time. The mind moves in space; the tongue enters space. Therefore the name Khechari {"moving through space"} was chosen by the masters for this mudra.
In his introduction, Akers notes that there has always been disagreement over which yogic activities should actually be put into practice (this disagreement is noted even in this pradipika), and emphasizes, "some practices in this book I don't recommend at all." In modern times (and quite likely five hundred years ago as well), surgical alteration of the tongue is seen as a fringe practice and is not accepted into the mainstream of yoga. That said, there it is, plain as day, in one of the most profound sections of one of the core texts.
Khecharimudra serves as an excellent example of the spiritual application of extreme body modification.