For trillions of years, Corizant was completely covered in city, with no natural resources to speak of. The citizens completely relied on imported goods to survive, and as planets began to die, the Corizanti found themselves in a bind. There were many attempts to harness wind and geothermal power as well as mine steel from the Underlevels, but more often than not these attempts did not go very well, and Corizant started to die.
However, after the arrival of the Force-Lords, Force-Energy was introduced, powering every piece of technology without emissions. However, the Force-Lords did not extend this benefit to much of the Underlevels, leaving them with old power sources of the past. Indeed, with Force Energy, the upper levels of Coruscant became a much cleaner place.
Fall of the Capital
Circa 700,000 ABY, the long-faltering Crasteline government fell after two business lords declared their long-held sections of the planet independent. Since all but one Force-user were eradicated in circa 482,000 ABY, the Democratic Alliance of the Galaxy only had a rudimentary militia, which was easily crushed by the business lords. The end for Crasteline came with the suicide bombing of the representative from Corellia, killing many representatives and destroying the Congress building. This bombing was revealed to be a plan of the allied business lords, who proceeded to hunt down every leader of the DAG. Within months, Crasteline split into 27 separate nations, which left all the other star systems in anarchy, forcing them to make governments of their own.
The Split Planet
For nearly 1 million years, Crasteline, now widely called Corizant by most of its citizens, remained split into several nations, many of which rose, clashed, and fell over time. Circa 1,700,000 ABY, Corizant was in a state of dystopia. The piling up of wasteful emissions resulted in the deaths of millions of people, and the population often had to have mutations to survive. This suffering continued for millions of years.
The Death of the Sun and Arrival of the Force-Lords
Circa 16,500,000 ABY, Corizant's sun, Corizant Prime, was fading, resulting in the planet becoming colder and the start of another mass extinction. However, one day people woke up to light and heat. It was announced that there were people at the highest skyscraper, people wearing robes more intricate than any known before. Every citizen from the 16 current kingdoms, which were all near stagnation, gathered to the people who brought warmth back to Corizant.