A youtuber going by the nick SKELUX recently made a video where he explained he wanted to discover the "minus" worlds of different NES games. He decided to start with the original The Legend of Zelda.
It is important to point out you cannot simply pop up the game and be able to access it. He had to check the actual code for several hours in order to manually remove the instructions put as a measure to prevent people from reaching it.
The name "minus world" comes because each couple of levels in Super Mario Bros. are called "world". You start in World 1-1 and keep progressing. However by causing a glitch it is possible to get into a warp pipe you aren't supposed to, which will make the game try to load the nonexistent world 36 -1. However this will be displayed for users as "WORLD -1".
To be frank with you, "world" is very loosely used here. Basically it is calling to a space in the memory that wasn't mean to exist. Because the game has to display something it will pull from the assets and build what you see. So that's basically breaking the game. There's most likely nothing you'll find that was intentionally done by the developers. For the first games it was common not to use negative numbers. Most games made sure in their code to prevent people from trying to load anything outside the boundaries.
That said, it can be pretty interesting to see the exploration of these quasi worlds. In his video, Skelux explains how the minus world works and gives a tour around. Prepare for a world where you can stab the old man and walk through walls.
Was the world anything close to what you were expecting?