There are certain caverns in Hoenn containing ancient, cryptic text. If you're able to decipher them, you'll be able to meet the three Legendaries Regirock, Regice, and Registeel. After catching the three of them, a powerful Legendary Pokemon will awaken!
Regirock
Category: Rock Peak Pokémon
Type: Rock
Ability: Clear Body
Height: 5'07"
Weight:507.1 lbs.
Regirock is a Pokémon that is made entirely of rocks and boulders. If parts of its body chip off in battle, Regirock repairs itself by adding new rocks.
Regice
Category: Iceberg Pokémon
Type: Ice
Ability: Clear Body
Height: 5'11"
Weight: 385.8 lbs.
Regice's body is made from ice from the ice age. It controls frigid air of –328 degrees Fahrenheit.
Registeel
Category: Iron Pokémon
Type: Steel
Ability: Clear Body
Height: 6'03"
Weight: 451.9 lbs.
Tempered by pressure underground over tens of thousands of years, Registeel's body cannot be scratched. Not only is it hard, this mysterious material also shrinks and stretches flexibly.
Regigigas
Category: Colossal Pokémon
Type: Normal
Ability: Slow Start
Height : 12'02"
Weight: 925.9 lbs.
Regigigas is said to have made Pokémon that look like itself from special icebergs, rocks, and magma. There is an enduring legend that states this Pokémon towed continents with ropes.
Reference and Image Source: pokemonrubysapphire