1. Trepanation

Ouch. In one of the oldest known medical interventions, a hole is drilled in the skull of a patient who is suffering from defects such as seizures or migraine headaches. The idea was to relieve pressure in the head which was believed to be causing the ailment. Today, trepanation is used on a very limited basis as a mechanism to access the brain for necessary surgery. Some people practice recreational or spiritual trepanation, presumably because they need modern medicine like they need a hole in the head.

2. Female hysteria
3. Expanding Earth
4. Phrenology
5. The Island of California

This list has been severely abridged. To read the full list, view the original post at it’s source:
5 Truly Bizarre (and Discredited) Historical Theories (Oddorama)