Gravity zones are strange areas where gravity seems to be reversed relative to the Earth. Some gravity zones even reverse an entity's gravity permanently upon entry, persisting until the entity enters another gravity zone. Ancient writings imply a society predating most modern intelligent species invented these zones, and used the permanent zones to quickly enable space takeoff, which accelerated their mass migration to a more hospitable planet.
The gravity reversal caused by gravity zones is not immediate, but a curve precisely calculated to cause as little disruption of internal organs as possible while taking less than 1/10th of a second.