Is there anything past the flagpole in Super Mario Bros?

Is there anything past the flagpole in Super Mario Bros?

As we know it is possible to jump over the flagpole in Super Mario Bros,
are there any secrets or easter eggs beyond them? Growing up I heard
rumors that there was a secret pipe if you ran far enough, however I'm
skeptical now that I'm older since it appears that it's more of an
unintended exploit than a "secret".
This question applies to the original or any remakes of the original.