Underground

You're new to the UNDERGROUND, aren'tcha?

Flowey, after introducing himself

The underground (JP: ちていのせかい) 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 terrain and climates, and features five main areas: the Ruins, the frigid and wintery Snowdin, the marshy and cavernous Waterfall, the blistery and barren Hotland, 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.

History

After the humans defeated the monsters in The War of Humans and Monsters, the latter was banished into the underground. A barrier was then erected to prevent insiders from leaving, though outsiders could still freely enter. Wary of humans, monsters retreated into the Ruins and adopted Home as their first capital, living normal lives in the underground from then on. Additionally, the Ruins was structured to contain diversions in the form of puzzles.

At an unspecified point in time, monsters documented the war, regarding SOULs and the Delta Rune's prophecy on plaques on Waterfall's walls.

One day, the fallen human ascended Mt. Ebott and tripped on a vine, causing them to fall inside the underground. The human was discovered by Asriel, Asgore and Toriel's son, and subsequently adopted into their family. The fallen human's presence brought hope to monsters. Eventually, no longer fearing humans, monsters ventured outside of the Ruins. They populated the rest of the underground, and made New Home their new capital.

Hotland, with the CORE in the distance.

In contrast to Home, some monsters were content to settle at different locations than in New Home. From Waterfall, monsters acquired modern technology and discarded items from the surface. At some point, the CORE was built by W.D. Gaster to generate electric power for the entirety of the underground.

One day, the fallen human grew ill and requested to see the golden flowers in their home village. After they died, Asriel absorbed their SOUL and became a powerful entity who could cross the barrier. The fallen human, sharing the body with Asriel, carried their corpse to their home village. The humans attacked Asriel and the human, under the assumption that Asriel had killed the human child. Refusing to retaliate, Asriel returned to the underground with the human's corpse. He died at New Home's garden, where he succumbed to his wounds.

Since then, Asgore became vengeful, declaring another war on humanity and plotting to escape the underground. He decreed all humans who fall into the underground be killed and their SOULs collected; with seven human SOULs, the barrier would be shattered. However, the brutality of this decision disgusted Toriel, who relinquished her position as queen and left Asgore.

One of the many puzzles found throughout the underground.

Asgore continued to rule the land on his own, while Toriel exiled herself into the Ruins. More puzzles were created across the underground to foil human attacks. New Home begun having issues with overcrowding. Over time, six more humans fell into the underground, all of whom Toriel attempted to shelter and care for. They all eventually left the Ruins and were killed, their SOULs collected by Asgore. At one point, the Royal Guard, led by Undyne, was formed to patrol the underground for humans. Alphys, the new Royal Scientist, surveys the underground through hidden cameras to control Hotland's puzzles, and to warn and evacuate monsterkind if needed.

An eighth human, the protagonist, had then fallen into the underground. Unlike their predecessors, the protagonist's determination carried them through many obstacles the underground had presented, even managing to befriend or kill many monsters. Eventually, the protagonist manages to reach Asgore, where he stands before the barrier. One of few events occur depending on the protagonist's actions:

• On the Neutral Route, the protagonist and Asgore battle. The latter is defeated, after which Flowey intervenes and absorbs the six human SOULs. The protagonist defeats the powered-up Flowey and is transported outside the barrier after the battle, leaving the underground. Sans then calls the protagonist and tells them the fate of the underground, which varies depending on who was killed.
• On the True Pacifist Route, Asgore is stopped by Toriel and the friends the protagonist made. Then, all of them are ambushed by Flowey, who absorbs the six human SOULs as well as the SOULs of most monsters in the underground, transforming into his true form, Asriel. However, with all the SOULs, Asriel gains compassion and ultimately loses the will to fight, instead using the SOULs' power to shatter the barrier. Asriel returns the SOULs, knowing that he will turn back into Flowey, and begs the protagonist to forget him. The monsters leave the underground and live peaceful lives on the surface.
• On the Genocide Route, certain monsters try to stop the protagonist's rampage while Alphys leads the evacuation. Upon reaching Asgore, the protagonist instantly defeats him with one blow and proceeds to destroy Flowey mercilessly. The fallen human appears before the protagonist and explains that their determination brought them back to life. With the protagonist's guidance, they realized that the true purpose of their reincarnation was to gain power. The fallen human then, with or without the protagonist's consent, uses that power to destroy the world.

Trivia

• The eight humans falling to the underground might be a reference to Alice in Wonderland.