Earth

Earth is a planet in the solar system that is divided by two different regions, the Underground and Surface, which houses its most prominent species, monsters and humans, respectively.

Location
It is not entirely stated where exactly Earth is located, but the sun's appearance when the barrier is broken places it in the solar system, which even more proven by Sans's comments about space and the sun.

Features
One of Earth's most distinctive features is the Underground and Surface, as well as the barrier that separates them. This barrier is included in Earth's more magical properties, which is the main source of one of Earth's native species, monsters.

Appearance
Earth seems to be fairly large, having enough room to fit the entire Underground below its surface.

The Battle
Humans and monsters peacefully rule over Earth, until the War of Humans and Monsters breaks out. After the war, however, the humans claim their victory and banish the monsters underground, sealing them in with a magical spell.

The First Human
In the year 201X, a human child climbs Mount Ebott. Upon reaching the top, they stumble upon a large cave and enter it, where they discover an enormous hole. They trip on a root and fall into the hole until they land, thousands of feet below the surface, in the Ruins.

Mount Ebott
Mount Ebott, located somewhere in the United States, has a cave at the top that is filled with vines and roots, where the only known entrance to the Underground is. There is a path that goes through the woods to get there, but the layout of this path is unknown.

Ruins
The Ruins starts at the Underground's entrance, a large cavern directly beneath Mount Ebott. The only light source in the room, the sunlight from the gigantic hole above, is shining down on a bed of golden flowers. Crumbling, marble pillars line the walls, vines climbing up them, and a small hallway branching off the east of the cave leads to a stone archway with the Delta Rune carved into the top.