In my 10 book series, the historical time line turns on the fact that the greatest nation in the major continent's history, disappeared overnight during a national celebration.
This nation was enjoying its Golden Age and was an industrialized-type nation on the verge of becoming technological.
The entire nation disappeared leaving only maybe 15 or 10 percent of unusable terrain. In my story, this nation was situated in the middle of the continent and left nothing that was familiar on the spot where it once stood.
There were no survivors nor any debris from their advancing culture left behind, and the many advances that they had made, many of which, they had recently accepted trade deals to share with the international community, were nowhere to be seen.
Right now, I have a lake of toxic fluid occupying the spot. But, could I have a kingdom-sized abyss? And, if so, how would this change the weather pattern?
The disappearance of this pivotal nation drove the remaining continent into a Dark Age.
Using what principles of Physics or geographical phenomena could I believably apply, to make an entire nation physically disappear? Since, this series uses this cataclysm as a central mystery, earthquake is too predictable.
I will accept if entire districts disappear simultaneously leaving only rocky outcrops or barren cliffs. So long as more than 80 percent of the nation is gone.