Ryugu-jo

In Japanese folklore Ryugu-jo (竜宮城, 龍宮城, "Dragon Palace Castle") — this is an underwater palace Ryujin, the sea dragon-kami.

Ryugu-jo is described in Urashima Tarot as located in a few days of swimming underwater. 

A wonderful gate appears in front of him. Different versions describe a palace made of gold, crystal, coral and pearls with sloping roofs that are visible above the gate. The large hall inside is illuminated by fish scales. The palace is surrounded by a garden, each of the four sides of which corresponds to different seasons: cherry blossoms in the east (spring), buzzing cicadas in the south (summer), colorful maple leaves in the west (autumn) and snow in the north (winter). 

Three days spent in Ryugu-jo are equal to three hundred years above the surface (in another version, three years of Ryugu-jo are equal to seven hundred years on the surface).