EBOTT
The Surface (or Ebott (fan)) is the region of the Earth where humans live separate from the monsters, who reside in the Underground. Both species are separated by the Barrier on Mount Ebott since The War of Humans and Monsters. In Mt. Ebott's cave has lengthy vines growing around its caverns, and is the only known point of entry into the Underground.
UNDERGROUND
The Underground is the primary setting of Undertale. It is a large region within Mount Ebott that is separated from the Surface of the Earth. It houses the kingdom of Monsters, who are ruled by King Asgore, and formerly by his ex-wife, Toriel. The Underground has varying climates, and features six different areas: the Ruins, the frigid and wintery Snowdin, the marshy and cavernous Waterfall, the blistery and barren Hotland, the technologically advanced CORE and the Capital. The Underground's exit is blocked by the Barrier, which was placed by the Humans long ago. Anything and anyone can enter the Underground by descending into Mount Ebott.
RUINS
The Ruins is first area in Underground. There are many puzzles and traps within the Ruins, as well as a surplus of red leaves and vines that are strewn throughout the area. A small river runs through the Ruins, and the dark ground is hallmarked by small patches of auburn leaves. Despite the abundance of red leaves in the Ruins, only a single black tree is seen here, which is in front of Toriel's Home.