Behind the waterfall there is a cave
After you got to the 255th floor of the middle class mines and got the sword of legend from the princess, you can go to the cave behind the waterfall. To do this you have to the sword equip, and then go to either rock by the waterfall and examine it by pressing A. After the little cut scene is over and the has rock have been destroyed, you can enter the cave. In the cave you will find a 12x9 field and like all in door areas, the time will have stopped.