The Cubic Ruins are an area accessible from the deepest section of the Grasslands Labyrinth in Ecstasy Grasslands.
The Cubic Ruins, as the name describes, are a large structure under a bright sky and over a large body of water, whose exterior is made out of concrete cubes with grass and some dead trees on some of its floors, along with trees showing up in the background.
Interacting with the lower edges of the cubes will let Mikan jump off them, allowing access to lower levels of the area, all the way to the lowest section. Jumping into the water near the lowest section will send Mikan back to the summit. Just near the area you start from, there is an egg which seems out of reach, which can only be obtained by entering an exit from the Catacombs, whose entrance is south of this egg.
The Catacombs serves as the Cubic Ruins' interior, and is a horizontally-looping labyrinthine area, consisting of a mossy floor and some water-filled pits, with broken bridges that Mikan can jump over upon interaction.
Going south of the entrance, then going east, and then taking the northern corridor, a tile with an eye can be found. When stepped on, this tile unlocks several exits and sends some Spider Fishes jumping out of the watery pits. By avoiding these creatures and reaching the northeast exit between two statues, you can enter the section of the Cubic Ruins holding the egg, which grants the Duckling Dress effect upon interaction.