Trepanning, also known as trepanation, trephination, trephining or making a burr hole, is a surgical intervention in which a hole is drilled or scraped into the human skull. This procedure is one of the oldest known surgical procedures, mainly performed by priests or "medicine men." Those who performed trepanation used materials such as stone, obsidian, bronze, or bone shaped into hand-held tools. The intentional perforation of the cranium exposes the dura mater to treat health problems related to intracranial diseases or release pressured blood buildup from an injury. It may also refer to any "burr" hole created through other body surfaces, including nail beds. The process to identify trepanation is through identification of a hole in the skull. The shape of the hole, cutting edges, and the healing process that is from the procedure also helps archeologists identify a trephined skull. A trephine is an instrument used for cutting out a round piece of skull bone to relieve pressure beneath a surface.