Sword Art Online Back Floors

Back floors:  

Essentially specially prepared areas for testing new and experimental content that were eventually revamped into hidden dungeons for more experienced players to find after being properly tested.

In other cases, they might be holding spaces generated by the system to carry special data not meant to be used in the main game, but are necessary for the system to properly function.

Please note that they usually  include a significantly raised difficulty level, riddled with puzzles and traps, and also in most cases do not follow the theme of the regular dungeon floors in the same region. The rules may vary, but they all contain drops and treasure of high value for those prepared and ready to plunge it’s depths.

For everyone who doesn’t quite get what back floors are, picture something like the <> from Hollow Fragment. Hard to get into, potentially dangerous if you are unprepared, and generally weird.