Barrier

The Barrier is a strong mystical force that separates the Underground from the Surface. It was put in place by the humans after The War, to keep the monsters trapped within the Underground, and the rest of humanity safe on The Surface.

Any human can fall through the barrier on Mount Ebott into the Underground, leaving them in The Ruins, where they are generally found by Toriel. However, only a being with the soul of a human and a monster can pass through from the Underground out to the Surface. Only eight humans have ever fallen through the barrier.