The Underground Ruins (Japanese: 地底遺跡Underground Ruins) is an area located within the Clay Tunnel.
The Legendary titans—Regirock, Regice, and Registeel—can all be found here, depending on which Key is active. Originally, the rooms of the ruins only provide hints on to how to open Regirock's chamber (which hint is available depends on if "the sun" or "the moon" is out), but fulfilling the conditions will change the great door into the entrance of Regirock's chamber and all other chambers activated by Keys.
Regirock can be found in the Rock Peak Chamber (Japanese: 岩山の間Rock Mountain Room), which is opened when the player presses the A-button after walking 6 steps down and then 9 steps right, from the center of the panel just before the great door. Registeel requires the Iron Key to unlock the Iron Chamber (Japanese: 鉄の間Iron Room), while Regice requires the Iceberg Key to unlock the Iceberg Chamber (Japanese: 氷山の間Iceberg Room). The Iron Key can only be found in Black 2 and the Iceberg Key can only be found in White 2; the key that cannot be found in one version can be sent through Unova Link from another game. A key can be used by selecting Unova Link followed by Key System after the title screen.
If the player has all three Legendary titans in their party, Regigigas can be awakened in Twist Mountain. It is not, however, necessary to use a trio caught in the Underground Ruins.
"Chamber of the one that joins the sun in protecting this place."
"Two answers are needed to find the true path. Courageous one, let me tell you the answer I know. Check the ground six steps down and ● steps right of the eyeball. The other answer can only be found when the moon is in the sky."
"The protectors were born out of rock, ice, and magma..."
During the night
"Chamber of the one that joins the moon in protecting this place.
"Two answers are needed to find the true path. Courageous one, let me tell you the answer I know. Check the ground nine steps right and ● steps down from the eyeball. The other answer can only be found when the sun is in the sky."
This location is based on the Sealed Chamber in Hoenn, and the music is a remix of that location's music.
At the northern part of the chamber, there are rock formations based on the dot patterns seen on Regirock, Regice, and Registeel.
All three dot patterns are present at once if a clue is available, but only the dot pattern for the Legendary titan found in the current chamber can be seen in the chambers that contain the Legendary titans.