Crumbling Island is a hidden area in Kingdom Hearts, it isn't accessible and neither appears in the game, but it can be accessed via Codebreaker or any other Hacking Device.
The place is made up of what it appears to be the Remainings of Destiny Islands, once swallowed by the Darkness.
The main color of this location is dark purple, the signature color of Darkness in the Kingdom Hearts series.
Various trees fall from Above, covered by a blue shining Substance, and with Crystals pouring from them, while the floor shows many fissures that shine on a blue coloration too. Walls rise from the cracked floor, and they show a moving smoke-like texture that appears in the light areas; from them, Orange Crystals pop out of the borders and fall from the upper parts.
If you go upper, crystal stars begin appearing, possibly symbolizing the Paopu fruit, typical of the Islands.
Ice Blobs emerge of the walls in the highest parts, and they melt, making little streams flow through almost every level of the Area.
The Ceiling shows a big hole, and across it, a shining blue sky with some clouds can be seen, in contrast with the dark look of the environment.