when your going to lilycove, on route 120, there will be some
stairs, walk up and you'll see some tall grass, walk through, there
will be more stairs. walk up and you'll see a guy, click A and
he'll tell you, "That giant rock... I would like to believe it may
indeed contain ancient ruins. but I see no entrance." you'll see an
entrance, go in. and the braille (dots) will read "those who
enherit our will, will shine in the middle." that means have a
Pokemon that knows flash, and use it in the middle, and voila.