The Origin of the Batcave
The Batcave is a central location in the Batman franchise. Its existence is purely accidental. Back in the days of the 1940s Batman film short serial, the producers wanted to have a hanger. They couldn’t afford one, but they did have a cave set lying around the studio. So in the second chapter of the series, the show unveiled the Batcave. A few months later, comic book artist Bob Kane worked it into the comic book series.
At the link, you can read about other plot elements of Batman that date back to the serials.