Super Mario Bros. · NES · 1985
A glitched level accessible through a specific wall-clipping trick that sends the player to a corrupted "World -1," an infinite underwater loop that cannot be completed.
World -1 is not a designed Easter egg but a programming artefact — a side effect of how Super Mario Bros. stores level data in memory. By clipping through the end-of-level pipe at World 1-2 at a precise position, the game reads the wrong memory address for the destination level, landing the player in a level whose index cannot be represented normally (displayed as a blank or a minus sign depending on the region). The NES version loops infinitely, while the Famicom Disk System version leads to a different set of corrupted levels. The trick spread through playgrounds and gaming magazines worldwide, becoming one of the first major gaming secrets to achieve truly mass cultural awareness.
In World 1-2, run to the end of the level and position Mario next to the pipe leading to the exit. Crouch-jump into the ceiling block directly above the pipe. If done correctly, Mario will clip into the wall. Move left and drop down into the pipe on the far left (not the intended exit pipe). This warps to World -1.