From Bulbapedia, the community-driven Pokémon encyclopedia.
The Sealed Chamber (Japanese: おふれのせきしつ Stone Chamber of the Proclamation) is a location in Hoenn. It can be reached by diving on Route 134. It is not a common place to visit, as it is well hidden under the water and through the currents on Route 134. Legends are told about the Sealed Chamber, but only few people from Pacifidlog Town know its true existence. It must be visited to unlock the caves that hold the three Regi Pokémon.
The first room contains ten mounds of earth, marked with braille that lists the alphabet. The braille on the door says to use Dig. This opens an entrance to the second room.
The chamber's history is shrouded in mystery. Not much is known about it though the translation of the braille in the second room is as follows:
|| Japanese (normal orthography)
| 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 waits.
| わたしたちわ このあなで くらし せいかつし そして いきてきた
すべてわ ぽけもんの おかげだ
だが わたしたちは あのぽけもん をとじこめた
ゆーきあるものよ きぼーに みちた ものよ
とびらを あけよ そこに えいえんの ぽけもんがいる
| First comes Relicanth.
Last comes Wailord.
| はじめに じーらんす おわりに ほえるおー
The last section refers to how the Desert Ruins, Island Cave and Ancient Tomb are unlocked. For an unknown reason, reading the uppermost inscription in the second room with Relicanth at the top of the party and Wailord at the bottom of the party causes an earthquake to occur that opens the doors. Note that the party positions must be reversed in Pokémon Emerald.
It is believed by some that the "We feared it." section refers to Regigigas, as opposed to Regirock, Regice, and Registeel, as it wouldn't make sense to refer to three Pokémon as an "it".
The significance of why Wailord and Relicanth are needed might have something to do with the general theme of the Ruby, Sapphire and Emerald games; the relationship between land and the sea. Relicanth is based on the coelacanth, a creature that is considered to be closely related to the ancestors of all land-living vertebrates. Wailord is based on the whale, a creature whose ancestors once lived on land but have since evolved to live in the ocean.
In the manga
In Pokémon Adventures, the inscription in the chamber is instead on a damaged stone tablet. The letter that Sapphire delivered to Steven from President Stone contained a reconstruction of the damaged paragraph. With the help of Sapphire, the Elite Four unseal the Desert Ruins, Island Cave and Ancient Tomb. This is the backup plan the Stones had made in the case that they were unable to destroy the Seafloor Cavern before Team Magma and Team Aqua could get to Groudon and Kyogre.
The Elite Four believe that the it in the phrase "We feared it" refers to the risk of trying to control a Pokémon that is too powerful.
In other languages