Castle in the Sky is an anime by Studio Ghibli. Its is a beautiful tale of a city lost among the clouds called Laputa, kept aloft by a crystal called Aetherium. It has been lost for so long that it is believed to be a ledgend but not to some and the two heroines Sheeta and Pazu race to reach laputa before the sky priates reach the castle and take the crystal to gain great power.

When I see images of Laputa it looks like a whole city has just been lifted out of the ground from where it was built. It looks made out of natural organic material until you see the dome at the bottom that looks very alien. That section houses the crystal and i supose it fits as the crystal could be considered an unnatural force.