Perhaps one of the most famous worlds of the High Middle Regions (Hinterworlds) is Vriin, famed for its beautiful Sky Islands. The planet is believed to have been visited at some point in the distant past by a travelling AI who, for reasons unknown, terraformed the planet and then abandoned it.
In the process of terraforming, the itinerant AI apparently employed gengineered bionano devices to manufacture carbon nanotube based vacuum dirigibles for the purpose of lifting and moving large loads. After the planet was abandoned, the bionano continued to function but was mutated in its programming (generating a compulsion to cluster together) by what may have been an exceptionally active period in the local stars sunspot cycle.
The results of this mutation, after several millennia of evolution, are the Sky Islands. Great clusters of the naturally growing vac- dirigibles float across the sky. Numerous ancillary and symbiotic organisms grow across the tops and sides of these huge creatures/machines forming entire floating ecosystems above the surface of the planet.
The Sky Islands are considered truly a wonder by all, although some also see them as a cautionary tale about what happens when one leaves ones toys lying about unattended.