Borderlands 3: How to get outside the map in the Atlas HQ map on Promethea

Stand on the lip of the elevator on the gap between the doors and hit the button and then move backwards and it should deposit you on the floor of the inaccessible area of the map and you can explore from there. If you fall off the map you'll fall for a very long time. You can jump into the base of the elevator like I did in this video to die or you'll have to exit the game to the menu and then reload to try it again.