"ten years during which we can only guess at what Lord Voldemort was doing…"
--Albus Dumbledore (HBP20)
After the death of Hepzibah Smith, Riddle vanishes for ten years. He travels far and wide, sinking deeply into the Dark Arts, consorting with the very worst of wizard-kind. He undergoes so many dangerous, magical transformations that when he resurfaces years later as Lord Voldemort, he is barely recognizable.