CORE

The CORE is a subregion of Hotland, found after the MTT Resort, connecting the rest of Hotland to New Home. It is a technologically advanced facility that provides magical electricity to the Underground. There is an elevator to New Home in the center of the CORE. The entire structure is designed so that the rooms inside can change positions and the layout can be altered.

Appearance
The CORE is an entirely mechanical complex that is largely blue and cyan with red accents. Purple paths and bridges link some rooms. The CORE appears to be the most industrial and modern region of the Underground.

The CORE has lavish decoration. Multicolored neon tubes serve as wall ornaments and embellishments. The floors of many rooms are engraved with intricate patterns. Lasers, both decorative and functional, dot the region. Despite the detail work, some parts of some walls and floors are chipped away.

Ozone, a byproduct of electrical power, is omnipresent below the floor level of the CORE. This explains why the CORE seems to be made of stainless steel, titanium, or platinum; ozone is highly corrosive to most organic materials.

The substance below the ozone quickly melts the ice thrown by the Ice Wolf. The ice supposedly cools the CORE down, though cooling does not seem required as the Ice Wolf does not throw ice after Snowdin's evacuation in the Genocide Route. Alternatively, the CORE could be a source of ozonated water, which cleans clothes, sanitizes food, and purifies drinking water. In this case, the CORE not being cooled is not hazardous; ozone simply decomposes into oxygen at high concentrations and temperatures.

Main Story
Before the events of Undertale, the CORE was built by the former royal scientist, W. D. Gaster.

Neutral Route/True Pacifist Route
After leaving the MTT Resort, the protagonist enters the CORE, and immediately Alphys becomes worried. There is an excess of enemies, the elevator is not functioning, and the pathway to the right of the elevator is completely missing. Alphys guides the protagonist to the left pathway instead and ends up making poor recommendations for the protagonist. She eventually stops talking to protagonist out of embarrassment. As the protagonist comes to the end of the CORE, they meet Mettaton once more. He reveals to the protagonist that it was he who hired monsters to attack the protagonist, as well as him that broke the elevator, removed the pathway, and hired everyone to kill the protagonist.

Mettaton explains that all of the encounters between him and the protagonist, as well as all of the puzzles the protagonist must complete, were designed by Alphys as a way for her to insert herself into the protagonist's story due to her admiration. Mettaton, however, got bored of this and by the start of the CORE began to plot legitimately against the protagonist, so as to cause incredible drama and excitement within his show. Likewise, him gaining the soul of the main character would allow him to prevent Asgore from killing humanity, and allow him to exit the barrier and become famous on the Surface- in front of "hundreds of millions" of fans.

After the protagonist defeats Mettaton, Alphys makes her way into the room and panics over the damage done to Mettaton: Right before leaving for the city, Alphys reveals that Asgore cannot allow the protagonist to leave, and for them to escape, they will have to kill Asgore and harvest his SOUL to be strong enough to pass the barrier.
 * If the protagonist spared Mettaton, Alphys reveals in relief that his batteries simply ran out. She reassures the protagonist, falsely reminding them that Mettaton is just a robot.
 * If the protagonist killed Mettaton, Alphys tries to conceal her pain from losing her friend. She lies to the protagonist that Mettaton can be replaced with another robot.

After this, the protagonist enters the elevator and leaves for New Home.

Genocide Route
After leaving the MTT Resort, the protagonist enters the CORE. The elevator is functional, and the door leading to New Home is unlocked, allowing the protagonist to continue straight on to Mettaton.

Mettaton makes his last stand, transforming into Mettaton NEO. The protagonist kills Mettaton in a single attack:
 * If the protagonist has not killed as many monsters as possible up to that point, the Genocide Route is abandoned as, according to a dying Mettaton, the protagonist was holding back. The protagonist may continue to New Home on the Neutral Route.
 * If the protagonist has killed as many monsters as possible up to that point, Mettaton makes a brief quip before dying, and the Genocide Route may be continued into New Home.

Trivia

 * The majority of enemies in the CORE are harder counterparts of enemies in the Ruins, which also appear in Hard Mode.
 * The enemies appear even if the kill counter is satisfied on the Genocide Route. They also count as a kill and reward extra EXP and GOLD.
 * Ice thrown by Ice Wolf from Snowdin travels through the Underground to the CORE, where it melts to cool it down.
 * The visual design of the CORE, as well as the fact its music continues through battles, was likely inspired by the Ocean Palace in Chrono Trigger, one of Undertale's major inspirations.