...the Polosomotopo Island. To get to this island, go to any patch of water where you can use dive, and click the A button forty-seven times, then click L, then click A another two times, then dive into the water. (Don't forget to bring a Pokemon with the Dive HM) Now you press R seven times, then use a rare candy on the Pokemon in your sixth slot. If that Pokemon was raised to level 86, the ground will open up. (If not, you have to repeat the cheat again.)
You enter the hole in the ground, and you will be taken in a current, and will end up in the middle of the pacific ocean. Use Sweet Scent, and if done correctly, Polosomotopo Island will rise out of the sea. Surf over to the island...