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- "‘The Barrier’. It’s what keeps the monsters in the underground away from the humans preventing them from harming us."
- ―Human History Volume 1[src]
The Barrier was a strong magical force separating the Underground from the Surface. The humans put this in place by using a magic spell after the War of Humans and Monsters to keep the monsters trapped within the Underground and the rest of humanity safe on the Surface.
Any human could fall through the Barrier on Mount Ebott into the Underground, leaving them in the Ruins, where Toriel generally found them. However, only a being with the SOULs of a human and a monster could pass through from the Underground out to the Surface. The Barrier could be broken, but seven human SOULs are required to do so. Only eight humans have ever fallen through the barrier: Chara, the six humans whose SOULs were collected by Asgore Dreemurr, and Frisk.
External Links
Barrier from the Undertale Wiki.