Overcity

Overcities (or sky cities) were large tower blocks the size of cities constructed on Earth during the 30th century. They had large anti-grav engines which allowed them to hover over the polluted, war-torn surface. Half of Earth's population lived in the overcities, the wealthier having access to the higher levels. The poor and aliens lived on the planet's surface in undercities. (TV: The Mutants, PROSE: Original Sin)