In Japanese mythology, Ryūgū-jō (竜宮城/龍宮城) is the undersea palace of Ryūjin, the dragon god of the sea. Depending on the version of the legend, it is built from red and white coral, or from solid crystal. The inhabitants of the palace were Ryūjin's servants, which were various denizens of the sea. On each of the four sides of the palace is a different season, and one day within the palace is equal to a century outside its boundaries. In legend, Urashima Tarō visited Ryūgū-jō for three days.
The Dragon Palace is based on Ryūgū-jō.