CORE

The  is the fifth region of the game, located inside Hotland past the MTT Resort, connecting Hotland and New Home. It's a technologically advanced facility that provides electricity to the Underground.

Appearance
The technological center of the Underground, the Core is an entirely mechanical complex that is largely blue and cyan.

Main Story
Before the events of Undertale, the CORE was built by W. D. Gaster. 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, etc. 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 legitimately plot 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, panics over the damage done to Mettaton, but reveals that it was only that his batteries ran out. 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.

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

Trivia

 * The majority of enemies in the Core are harder counterparts of enemies in the Ruins, which also appear in Hard Mode.
 * 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.