For this to work you will need a Wailord in your first position and Relicanth in your last. From Pacifidlog Town surf southwest in the ocean currents to Route 134. You will find a place called the Sealed Chamber. When you get there dive go underwater till you see what appears to be a room. Go to the bottom of the square room read the visual braile. It say go up here, dive now. You will enter the room it says Dig Here. That means you'll need a digging Pokemon. Go into the next room. The rest is self-explanatory if you have Wailord in your first position and Relicanth in your last.
If you need help finding the Regi's caves or knowin what to do in the caves PM me
