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Hellmouth referred to a sort of portal between Earth and several hell dimensions. For this reason, the Hellmouth's area was a hot spot for supernatural activity, sending out mystical energy that affected the nearby population, and attracting supernatural creatures even when closed.
They were created when a Deeper Well or Wellspring was 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. Opening a Hellmouth posed apocalyptic threats predicted by portents, allowing demons to cross over to Earth. One such demon included a tentacled beast with multiple heads and Turok-Hans.
The Sunnydale Hellmouth laid underneath the library of Sunnydale High School, under the principal's office in its rebuilding. The Seal of Danzalthar was the gateway, located in the school's basement. Buffy Summers, the last guardian of this Hellmouth, identified that its activity usually got apocalyptic proportions around May.
There were attempts to open the Hellmouth by the Master in 1997, the Sisterhood of Jhe in 1999, and a Sacrifice of Three performed by three Vahrall demons in 2000.] It was effectively opened by the First Evil in 2002, Lissa in 2003, then by Buffy, Faith, and a group of Potential Slayers again in 2003.
Spike used a magical amulet to destroy the First's Turok-Han army (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.
There were also hellmounth in Cleveland and Easter Island.