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Routes 132 - 134 are west of Pacifidlog. Bring a Wailord and Relicanth in the first and last spots in your party, and a Pokemon with Dig, because on Route 134, if you traverse the currents right, lies a Dive spot leading to the Sealed Chamber (but feel free to collect a Horsea and a Rare Candy among other items around this area here beforehand!)
Before you lies a couple of Braille inscriptions. You use Dig in the first entrance of the chamber, and Wailord and Relicanth do the rest of the work for you in the second chamber. This awakens the legendary titans with their own unique little braille puzzles to do across Hoenn. Capture them whenever you like!
It's been said the reason Wailord and Relicanth are the requirements here because Relicanth is based on a coelacanth, a fish closely related to the ancestors of the first land-dwelling vertebrates, while Wailord is of course based on whales, mammals whose ancestors originally dwelled on land before evolving to thrive in water. This land-water duality is the symbolism behind much of Hoenn's lore and secrets.