The Hellmouth

In Buffy the Vampire Slayer, the Hellmouth is a subterranean mystical portal "between this reality and the next" and a convergence point of mystical energies, lying directly beneath Sunnydale High School. It attracts evil forces, which the series uses to explain why vampires and demons are so common in Sunnydale. The Hellmouth existed at the site long before humans settled the area; Richard Wilkins founded the town on top of it in the late 19th century under a pact with the demons, in return for a promise of immortality. Sunnydale High was rebuilt over the Hellmouth at the start of the seventh season, and the final battle with the First Evil left the town collapsed into a giant pit, closing the Hellmouth's entrance.