Note : Before bothering with the Sealed Cavern, you’ll need Pokémon with Dive and Surf. More importantly, you’ll also need to bring Relicanth and Wailord , plus a Pokémon that knows Dig.
Relicanth can be found in the underwater seaweed while diving, although it’s rare. Meanwhile Wailord can be evolved from Wailmer, who’s found commonly by surfing in the seas. Lastly, the TM for Dig can be found in Route 114.
|Magikarp||Water||Old/Good Rod||65% (Old), 60% (Good)|
|Tentacool||Water/Poison||Surf/Old/Good Rod||65% (Surf) 35% (Fishing)|
|Wailmer||Water||Good/Super Rod||5% (Good), 60% (Super)|
Assuming you’ve been to Route 133 once already, Fly back there. Upon arrival, Surf to your west from your initial island. When you land again, head past the trainers and surf from the west of this island.
When you land on the shallow-water island, Surf west, ensuring your path takes you to a current heading straight west and into the calm waters. Dive into the deep water below, then go south an area.
Go south for a while until you see a big pillar and some light. Near the light, examine the rock pillar to the north, which has some … ah, Braille. It reads "GO UP HERE". Ascend to the surface, then.
here’s a bunch of Braille-engraved rock walls in here. They read, from left to right, top to bottom, the Braille alphabet. (The period and comma are the rightmost two and the smallest walls.)
The northern wall will read "DIG HERE". Use Dig there; simply teach it to someone if you must, preferably to replace a TM move (so it can be re-replaced). In our case, it was Pikachu’s Return. Instead of leaving, a door opens. Go through.
The Braille through the next room’s rock pillars recounts a minor story: "In this cave we have lived. We owe all to the Pokémon. But, we sealed the Pokémon away. We feared it. Those with courage, those with hope. Open a door. An eternal Pokémon awaits."
Head to the north side of the room and read the message there: "FIRST COMES RELICANTH. LAST COMES WAILORD." If you need to rearrange your party so that Relicanth is the lead and Wailord is the bottom-right one, do so and examine it again.
When you do this, the ground will shake violently and it will sound like three doors opened up. That done, get out of here, redo your party if needed, and Fly to wherever you want to go next.
This minor side-sidequest is now over; if you want to see more, see the Legendary Pokémon page in the Extras chapter.