In 1968, Bob Gibson had one of the best single-season performances by a pitcher in baseball history. His 1.12 era was the lowest in the modern era. His excellence in a season of pitching excellence (22 other pitchers had a sub 2.00 era that season) resulted in baseball lowering the mound.
Gibson was not only dominant. He was intimidating. He owned the inside half of the plate. Batters, reportedly, feared to step into the box against a heater that he could locate with high velocity and command at their knees or chin.
Ask anyone from that era.