Almost immediately after Superman first appeared in 1938’s Action Comics #1, other publishers took note of the character’s instant popularity, and rushed to produce similar characters of their own, giving rise to both the term “superhero” and to an entirely new genre of speculative fiction. Many of these publishers went out of business. Others failed to provide proper copyright notices, or to renew their copyrights, as required under the copyright laws in effect at the time. The laws have changed in the intervening decades, but those older works had already fallen into what is known as the public domain.
Geeking Out with Jeffrey Harlan