A floating island or rock in the air.
Laputa's means of propulsion and altitude control is detailed below.
I turned back and perceived a vast Opaque Body between me and the sun, moving forwards towards the island; it appeared to be about two Miles high, and hid the Sun six or seven minutes.
... the Reader can hardly conceive my Astonishment, to behold an Island in the Air, inhabited by Men, who are able (as it should seem) to raise, or sink, or put into a Progressive Motion, as they pleased.
From Gulliver's Travels, by Jonathan Swift.
By means of this Loadstone, the Island is made to rise and fall, and move from one place to another. For, with respect to that part of the Earth over which the Monarch presides, the Stone is endured at one of its sides with attractive Power, and at the other with a repulsive. Upon placing the Magnet erect with its attracting end toward the Earth, the Island descends; but when the repelling Extremity points downwards, the Island mounts directly upwards. When the Position of the Stone is obliqe, the Motion of the Island is too.
By this oblique Motion the Island is conveyed to different Parts of the Monarch's Dominions.