How do you get behind the waterfall in Harvest Moon DS Cute?

You must get the Legendary SWord, equip it, and press the A button while standing in front of the boulder blocking the path.

The SWord is at the bottom of the second mine (there are 255 floors). Once you get to the bottom floor defeat all the monsters and you will gain entrance to the SWord room.

You will not be able to pick up the SWord unless all your Tools are upgraded to Mystril.