Before Lord of the Rings, there was the Chronicles of Narnia, written by Tolkein's fellow Inkling C.S. Lewis. I read them over again when I was a child. My favourite was the origin story, The Magician's Nephew. It featured a wood between worlds where you could, by stepping into one or another of its many shallow pools, access alternate universes.
In the late 1960s, I loved to watch The Time Tunnel, a briefly-lived series created by Irwin Allen in which, as the opening narration of each episode put it:
"Two American scientists are lost in the swirling maze of past and future ages, during the first experiments on America's greatest and most secret project, the Time Tunnel. Tony Newman and Doug Phillips now tumble helplessly toward a new fantastic adventure, somewhere along the infinite corridors of time."Murray Leinster, widely credited with the invention of parallel universe stories, wrote the tie-in novel.
Where do science-fiction writers get their crazy ideas?