Trepanation, also known as trepanning, trephination, or burr holing, is a surgical intervention in which a hole is drilled or scraped into the skull. It is one of the oldest surgical procedures known to humanity, dating back to ancient times. The procedure is used for epidural and subdural hematomas, and surgical access for certain other neurosurgical procedures, such as intracranial pressure monitoring. In modern times, trepanation is performed by neurosurgeons using diagnostic imaging to pinpoint the issue within the skull.

Trepanation has also been performed voluntarily by people interested in "enhancing mental power and well-being" since the early 1960s. Some people believe that trepanation can emulate the "third eye" and lead to clairvoyance or enlightenment, or even maintain a "permanent state of euphoria". However, the practice of trepanation is not supported by scientific evidence and is considered pseudoscientific.

Trepanation can also refer to the process of creating a hole in the fingernail or toenail, which is used as a procedure for subungual hematomas. Additionally, trephination has been documented in the treatment of glaucoma, which consists of using a 1-2mm blade and topical anesthetic to relieve pressure of the eye.

In conclusion, trepanation is a surgical procedure in which a hole is drilled or scraped into the skull. It is used for epidural and subdural hematomas, and surgical access for certain other neurosurgical procedures. While it is one of the oldest surgical procedures known to humanity, it is not supported by scientific evidence when performed voluntarily.