1. Before you attempt to recreate this bug, make a new world in Creative Mode, with Host and Player Options on. Then make yourself invisible, as this will also make you invincible.
2. Go The End by creating an End Portal. Set your spawn below The End.
3. Undo the first step so you will respawn at the same coordinates in the Overworld. Next, make yourself invincible again so you don't die in the Overworld.
4. Grab an empty map from the Creative Inventory, then fill it out. Notice how you are below the world and can see through various empty spaces (like caves and ravines). You are also permanently stuck under there.