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The term Sky Island (空島 Sorajima?) can refer to any island located in the sky. Known Sky Islands include:
Urouge's homeland is a sky island, although the name of the island has not yet been stated.
At some point, Bellamy went to Skypiea and returned with a pillar of gold.
Ballon Terminal, a ruined city located on a Sky Island in the New World. It appears to at least partially be suspended by a multitude of balloons along its underside.
The existence of sky islands are largely mocked or ignored by many. To those living in the Blue Sea, the idea of an island in the sky seems illogical, impossible, or unbelievable. Traveling to a sky island is difficult due to their location, although not entirely impossible. One way is to ride a current known as the Knock Up Stream. Gan Fall mentioned another way to get to the sky Islands, which he called the "Summit of High West" (ハイウェストの頂 Hai Uesuto no Itadaki?), but it is unknown as to what it actually is.