Trepanning (sometimes Trephining) was a basic surgical operation carried out, predominantly by medicine men, in prehistoric societies across the world,[19][20] although evidence shows a concentration of the practice in Peru.[16][19][21] Several theories question the reasoning behind trepanning; it could have been used to cure certain conditions such as headaches and epilepsy.[22][23] These might have been considered effects of an evil spirit trapped within a person. Despite it being medically inappropriate in the vast majority of cases and a very dangerous procedure there is evidence discovered of bone tissue surrounding the surgical hole partially grown back, so therefore survival of the procedure did occur at least on occasion.[16]