Hellmouth

A Hellmouth is an area of massive supernatural activity. Hellmouths are places of increased supernatural energy. According to the mythology of the "Buffyverse", this is the area in which the barriers between dimensions are weak. The Hellmouth has a focal point which serves as a portal between earth and the underworld. For these reasons, the Hellmouth attracts demons and other supernatural creatures, becoming a "hot spot" for supernatural activity.

The Sunnydale Hellmouth lies underneath the library of Sunnydale High school. The Seal of Danzalthar is the gateway, under the library in the old school and below the principal's office in the new school. There have been many attempts by apocalyptic individuals/groups to open or utilise the Sunnydale Hellmouth, such as in the episodes "The Harvest", "The Zeppo" and "Doomed". In season 7, The First Evil was going to open it to bring forth the Turok-Han, but Buffy and the potential slayers descend through the portal in the series finale, "Chosen" to fight them. Spike uses a magical amulet to destroy the Turok-Han and close the Hellmouth. The ensuing cataclysm not only collapses the Hellmouth, but swallows the entire town of Sunnydale along with it.