Trepanation is the practice of making a hole in the skull to expose part of the brain covering, for medical or mystical purposes.

That's not the bizarre part.

Trepanation has been carried out for thousands of years. Archaeological remains from the Stone Age onwards show evidence of human skulls with holes bored in them.

That's not the bizarre part.

It was done by pre-technological civilisations all across the globe.

That's not the bizarre part.

From the regrowth of bone on many ancient trepanned skulls, it is clear that many, if not most, patients who underwent the procedure survived.

That's not the bizarre part.

Many of these survivors show multiple healed trepanation wounds indicating they underwent the procedure successfully several times.

Okay, you can wig out now.

It seems to have been done to cure various maladies afflicting the head, such as migraines, epilepsy, mental illnesses, and of course spiritual possession. In some cases it may actually have been helpful, since modern surgeons perform similar operations these days (under anaesthetic!) to treat haematoma (blood build-up) in the brain caused by head trauma.