I forgot to add that in the last episode of Buffy, "Chosen," the Hellmouth was collapsed in on itself due to a mystical amulet worn by Spike (souled vampire who searched to regain his soul because he wanted Buffy to love him). Thus, the entire town collapsed in on itself. None of the residents were aware of the evil of the town on a conscious level, but they all knew to get out of town whether they realized why or not. That's the crater that's spoken of in the story.
In order for the good guys to defeat the bad guys all potential slayers had to be "chosen" by a spell. In the show, the history is something like, "Into every generation, a Slayer is born. One girl in all the world, a Chosen One, one with the strength and skill to hunt the vampires, to stop the spread of their evil ways, to cease their destructive manners, to prevent the end of the world. When one Slayer dies, the next one is called." To stop the overwhelming forces of evil that they were fighting, Buffy came up with the plan to awaken every potential slayer (in each generation many potential slayers are born, but only one will become a slayer when the previous one dies - it's a mystical thing, no one knows how the next slayer is chosen). Lois is one of the potential slayers, one who would most likely never have been called had it not been for the spell performed by Willow Rosenburg. Slayers were normally called at an average age of 15 (when the slayers were created waaaayyyy back in the day, the average life expectancy was maybe 40 years, so 15 was pretty much an adult).
