Sunnydale was a small town in California where Buffy and her friends used to reside, now its reduced to nothing but a giant crater. To the world, the events that caused Sunnydale's destruction was simply a horrific natural disaster - a monstrous earthquake that created the effect of a huge sinkhole. To the world, the residents of Sunnydale were able to evacuate because they predicted this earthquake would happen.
After the town's destruction, news media interviewed residents - some of them swore up and down that there were supernatural forces at work but many of their stories were ignored as crazy talk. After The Great Revelation , however, their stories were revisited.
What really happened, of course, was The First Evil 's attempt at raging war on the world by sending its army after Buffy and her then potential slayers. However, the army was defeated when vampire Spike sacrificed himself by wearing an amulet - this amulet had the power to wipe out the army along with him. The energy was so strong that it destroyed everything, including the Hellmouth that the town rested on.
This "natural disaster" was a historical moment and a point of national pride, since the evacuation process saved so many lives.