Laputa is a flying island described in the 1726 book Gulliver's Travels by Jonathan Swift. It is about 4.5 miles in diameter, with an adamantine base, which its inhabitants can maneuver in any direction using magnetic levitation.

Laputa was located above the realm of Balnibarbi, which was ruled by its king from the flying island. Gulliver states the island flew by the “magnetic virtue” of certain minerals in the ground of Balnibarbi which did not extend more than four miles above, and six leagues beyond the extent of the kingdom. showing the limit of its range. The position of the island, and the realm below, is some 5 days journey south-south-east of Gulliver's last known position, 46N, 183E (i.e. east of Japan, south of the Aleutian Islands) down a chain of small rocky islands

Laputa is described as being exactly circular and 4.5 miles (7.2 km) in diameter, giving an area of 10,000 acres (4,000 ha). It was 300 yards (270 m) thick, and comprised a bottom plate of adamant 200 yards thick, above which lay "the several minerals in their usual order", topped with "a coat of rich mould 10 or 12 ft deep".

More Info: en.wikipedia.org