The Hellmouth is a hellish portal and an area of negative spiritual energy within the Buffy and Angel series, although often referred to as "the Hellmouth" there are more than one such portals in existence - the largest and most dangerous however is located underneath what was once Sunnydale.
The Hellmouth is the reason Sunnydale has such a high population of vampires, demons and other-worldly monsters, who are attracted towards the negative energies the Hellmouth gives them.
Referenced from very early in the series the Hellmouth itself is never directly faced though the gang has one rare occassions faced a demon known as the Hellmouth Spawn, which appears to be a Lovecraftian Horror, connected to the Hellmouth.
- The word "Hellmouth" is derived from the original Spanish name for Sunnydale: Boca del Infierno, translated as "Mouth of Hell". While in this instance it refers to the city as a whole, in the Scoobies' usage, it has come to refer to the center of this mystical convergence.
- On an episode of Saturday Night Live, John Larroquette portrayed Bob Vila on This Old House, remodeling the Amityville Horror house which included a "garden variety Hellmouth," which he suggested converting into a fireplace.
- In episode 10 season 2 of Eureka, one of the characters suspects that "GD was built over a Hellmouth.".
- Hellmouth is the name of Oliver Morris' science fiction shop in Coupling.
- In the play Zombie Town by Tim Bauer, one of the theories given for the zombie uprising in Hayward, TX, is that the town is built over a Hellmouth. A character notes that both Cleveland and Sunnydale are built over Hellmouths, the latter being "well documented".