You're new to the UNDERGROUND, aren'tcha? The Underground is a large region underneath the Surface of the Earth, which houses the kingdom of Monsters. Exit from the Underground is blocked by the Barrier, which was placed by the Humans ages ago. Humans access it, the protagonist included, by way of a hole in Mount Ebott. It is ruled by King Asgore, and formerly by his ex-wife as well, Toriel. Spanning several different climates, the Underground features the Ruins, the cold and wintery Snowdin, the wet and cavernous Waterfall, the blistery and barren Hotland, the technologically advanced CORE, and the Capital.