complete the game with ... i think ... according to memory... 60+ pokemon owned, and Oakie will give ya the nash dex, as well as ask you to goto island 4 in order to fill in the dex... =D once thats opened, you can breed and even catch larvitar XD ...
... hmmm the unknown cave should've been opened after you've obtained both ruby and sapphire pi... oic the sapphire's stil on the other islands...