A Hellmouth was an area in which the barriers between dimensions was particularly weak, allowing the creation of portals between Earth and several hell dimensions. For these reasons, the Hellmouth attracted demons and other supernatural creatures, becoming a "hot spot" for supernatural activity. A Hellmouth is created when a Deeper Well or Wellspring is used to cut a path to another dimension and allowing its magic to flow through. 
Distortions of the laws of physics were made possible by the Hellmouth's influence, such as Billy Palmer's nightmares intruding into reality and the ignored of notable monsters such as the Master, the She-Mantis, Moloch the Corruptor, Machida, Ovu Mobani, Balthazar and even the famous Dracula. A Suvolte Demon also came to Sunnydale with the intent on breeding. According to Willow Rosenberg, the Hellmouth was responsible for the many churches located in Sunnydale as it made people "pray harder". Spike took a severely weak and injured Drusilla to the Sunnydale Hellmouth in hopes that it's energy would restore her to full strength, though it didn't work. It was credited for the hysteria that gripped Sunnydale following the First's uprising, causing most of the population to flee and the Sunnydale police to resort to vigilante actions. A Hellmouth also can become so strong that vampires could enter human residence without an invitation, and werewolfs could become more violent than usual
The Sunnydale HellmouthThe Sunnydale Hellmouth laid underneath the library of Sunnydale High school. The Seal of Danzalthar was the gateway, under the library in the old school and below the principal's office in the new school. Spike used a magical amulet to destroy the Turok-Han (as well as himself) and close the Hellmouth. The ensuing cataclysm not only collapsed the Hellmouth, but swallowed the entire town of Sunnydale along with it. As of spring 2003, the Sunnydale Hellmouth was permanently closed and inactive.
Attempts at Opening the Hellmouth
- First attempt by The Master in which he became "stuck" in it.
- Opening by The Master
- Opening by the Sisterhood of Jhe. 
- Ritual performed by three Vahrall Demons.
- Opening of the Seal of Danzalthar by the First Evil.
- Second opening of the Seal of Danzalthar by Lissa.
- Third opening of the seal of Danzalthar by Buffy, Faith and all the potential slayers.
Known Hellmouth locations
- Sunnydale, California (destroyed)
- Cleveland, Ohio
- Los Angeles, California (artificial mini-Hellmouth, destroyed)
- Easter Island, Chile
- Providence, Rhode Island  (non-canon)
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.