“Flash of Two Worlds!”
The Flash #123, Sept 1961 What happened? Attempting to appear invisible by vibrating his molecules at high speed, the Silver Age Barry Allen Flash is instead transported to another dimension, home of Golden Age Flash Jay Garrick, who Allen had always thought a fictional figure. Why was it shocking? It introduced Earth-Two, and the concept of the multiverse, to DC Comics; Golden Age heroes start to crop up everywhere, and things soon became very complicated indeed.