Sunnydale Hellmouth
The Sunnydale Hellmouth lies underneath the library of Sunnydale High school during the first three seasons of the show, until the School's destruction at the end of Season 3. In the Season 4 episode "Doomed" the Hellmouth is seen as a narrow but incredibly deep pit in the ruins of the school. When Sunnydale High is re-built at the beginning of the 7th Season, Xander Harris says that the new principal's office is now located directly over the Hellmouth.
In the 7th Season episode "Conversations with Dead People," a never-before-seen artifact, the Seal of Danzalthar, located in the school's basement below the principal's office, covers the opening of the Hellmouth.
There have been many attempts by apocalyptic individuals/groups to open or utilize the Sunnydale Hellmouth, such as in the episodes "The Harvest", "The Zeppo" and "Doomed".
Read more about this topic: Hellmouth (Buffy The Vampire Slayer)