Only a few have survived the Calamity; most are turned into petrified ashes that disintegrate upon contact.
Areas that survived the Calamity were only able to do so because of the presence of a local Core. Parts of the Wilds have also survived, perhaps because of the minimal human contact, apart from seeded Shards.
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In reality, the Calamity was the result of an official Mancer project. In the event of another Ura-Caelondian conflict, this project would have been initiated as an emergency "final solution", ambiguously described as "redirecting the Tazal Terminals to the stars."
The Calamity itself is attributed to Venn, a brilliant Ura Mancer and Zia's father. Horrified by the Mancers' genocidal intentions, Venn sabotaged the project to backfire on the Caels when it was deployed. He detailed all he knew in a journal and resigned from his post but was forced to return to work after his daughter was framed for treason. Rucks suggests that Venn himself was also forced to activate the project.
Venn's last words to his daughter were a warning, which suggests that he knew the project was about to be completed. Because the Ura had continued to abide by the post-War treaty, it is heavily implied that the pretext for the Mancers' project was little more than an excuse.