The Mushroom Kingdom posts:
Deep within Super Mario Bros. 3 lie hidden, unused levels. Some of them seem polished, with working goals and plenty of enemies, while others are devoid of enemies, lacking exits, or too short to be full levels.
Whether they evolved into other levels, or were merely used for testing, the “lost levels” provide an interesting glimpse into the development of Mario 3.
The provide additional links to a ROM patch that allows gamers to easily access these previously hidden levels.