Early in the morning at 6 AM when you first wake up, step outside the house and immediately come back in. Inspect your clock to see if it says "6:01." If it does, inspect the set of records on the shelf to begin a very challenging memory game.
On any level in the mines, save your game and turn off the DS, then restart it and load your data.
You will load onto the floor you saved on, but all monsters will be GONE.