Mount Ebott

Mount Ebott, sometimes stylized as Mt. Ebott, was a mountain on the surface of the Earth, very likely somewhere in the United States. It was the only known point of entry into the Underground, where monsters were sealed after the war against humans. Due to this, there was a legend that those who climbed the mountain never returned; eight humans fell through the barrier on Mount Ebott into the Underground.

Trivia

 * "Ebott" spelled backwards was "ttobe," a derivation of Toby. This was a hint towards Toby, the creator of the video game Undertale in both the real world and in this universe.