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. The barrier, however, only works one way. Only a being with the soul of a human and a monster can make it from the Underground out to the Surface, but any human can fall through the barrier on Mount Ebott into the Underground, leaved them in the Ruins, where they are generally found by Toriel. Only Eight humans have ever fallen through the barrier.