The CORE is a sub section of Hotland, accessible from the MTT Resort. 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.
- Alphys directs the protagonist throughout the area, the former slowly leaking more and more sentimentality with her time together with the latter.
- Various CORE-exclusive monsters appearing to be "black sausages" encounter the protagonist, all in fixed locations and orders.
- The protagonist can witness the ice thrown by Ice Wolf melting in the CORE.
- The protagonist fights Mettaton in a dark room, the latter broadcasting the action to his audience and presenting the battle as the grand finale.
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 the protagonist out of embarrassment.
Progression to the end of the CORE may be unlocked two ways; the sage's path, which consists of the protagonist solving a shooting puzzle, and the warrior's path, which consists of going through several encounters while approaching a button that activates the elevator. A third way to open the bridge to the end of the CORE is traveling there and reading the sign right before it, saying "I cannot fight. I cannot think. But, with patience, I will make my way through." If the protagonist stays in the area for long enough, the bridge opens.
After either of these paths are taken and fulfilled, the elevator to the end of the CORE will work. 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, broke the elevator and rearranged the pathways in the CORE.
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 protagonist’s SOUL would allow him to prevent Asgore from destroying 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:
- If the protagonist spared Mettaton, Alphys reveals in relief that his batteries simply ran out. She reassures the protagonist that Mettaton is just a robot.
- If the protagonist killed Mettaton, Alphys tries to conceal her pain from losing her friend, saying Mettaton can be replaced with another robot.
Right before leaving for New Home, 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.
True Pacifist Route
Monster encounters in this area stop after receiving (or after receiving the call to obtain) Undyne's Letter. NPCs that take the form of "black sausages" appear and reveal that Mettaton hired them to kill the protagonist, but quit since he never paid them. They also reveal that the CORE was rearranged in anticipation for the protagonist's arrival.
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 aborted as, according to 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.
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. However, cooling may not be 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.
CORE Approach to CORE Branch
The entrance to the CORE is a vertical room with the color scheme of the MTT Resort gradually fading into the color scheme of the CORE. Here, the protagonist witnesses two "black sausage" NPCs backing into the CORE. A surprised Alphys calls the protagonist and tells them that monsters are not normally supposed to be in the CORE.
The protagonist continues into the lobby, which has an elevator to the north and two paths to the left and right that veer upwards. Alphys directs the protagonist to the elevator, which is disabled. She then directs the protagonist to the path on the right, which is a small, square room with a fire pit to the north that is inaccessible to the protagonist. Going through the left path, which leads to a rectangular room with a doorway at the end, a monster appears and attacks the protagonist.
Continuing forward, the protagonist enters a room with a bridge that is cut off by a force field. Activating the switch near the force field deactivates it, but it also sends a group of lasers in a specific order. Alphys tells the protagonist that the order is "orange, orange, blue", but once the switch is activated, the order that the lasers come in is inverted (blue, blue, orange). After Alphys blames herself for messing up, the protagonist continues into a crossroads with a path to the north and a path to the east. Alphys directs the protagonist to the east, but changes her mind and tells the protagonist to go north. The northern path leads to a square room that contains another monster that attacks the protagonist.
The eastern room contains a bridge with many blue and orange lasers followed by a massive wall of blue lasers. Since Alphys is unable to turn off the lasers, she decides to cut the power for the entire room. When that happens, the way back is blocked by a "battery powered" force field. When the protagonist walks into the wall of blue lasers, the power suddenly turns back on, forcing the protagonist to stop. Alphys is unable to keep the power off, so she instructs the protagonist to move, then stop when the power turns back on. After crossing the bridge, the protagonist continues into the "Core Branch".
CORE Branch to New Home Elevator
The "Core Branch" is a four-way intersection with a SAVE Point in the middle. The signs in the room inform the protagonist that the northern path and the western path lead to the "warrior's path" and the "sage's path" respectively, and the east leads to "The End". Alphys calls the protagonist, completely at a loss for how to guide them, and eventually stops talking to them out of embarrassment.
Starting from the western path of the "Core Branch", the protagonist enters a room that veers upwards. Here, the protagonist can witness the ice thrown by Ice Wolf travel across a conveyor belt and melt into the ozone pit. Continuing north, the protagonist comes across a vertical room with a western path leading to a shooting puzzle similar to the ones in Hotland's Reactivated Door Puzzles. Solving this puzzle unlocks the path to “The End”. The northern path leads to a crossroads with an eastern path and a northern path. The northern path leads to a room with a trash can containing a Glamburger, while the eastern path leads to the middle portion of the "Core Branch".
Starting from the middle path of the "Core Branch", the protagonist enters a vertical room with an eastern path leading to the eastern portion of the "Core Branch". There is a sign on the path that tells the protagonist to "get lost... And stay that way." Continuing north, the protagonist encounters a four-way crossroads with the eastern and western paths leading to their respective portions of the "Core Branch" and a northern path that leads to the "warrior's path". The warrior's path is a horizontal bridge in which three monster encounters take place leading up to a switch at the end of the room. Activating the switch unlocks the path to “The End”.
Starting from the eastern path of the "Core Branch", the protagonist enters a room that mirrors the room in the western portion of the "Core Branch", except without the ice. Continuing north, the protagonist comes across a four-way crossroads. If neither the warrior's path, nor the sage's path are completed, the eastern path is blocked by a force field that eventually disappears after waiting long enough (hinted by the sign next to it). The western path leads to the middle portion of the "Core Branch", and the northern path leads to another crossroads, again mirroring the room in the western portion of the "Core Branch". The northern room contains a trash can with 100G inside.
After unlocking the way to "The End", the protagonist enters a horizontal room with a bridge and then enters the "Core End", which contains a SAVE Point, a doorway, and an elevator leading back to the lobby. The doorway leads to a dark room with Mettaton in the center. After Mettaton locks the door, the room suddenly rockets upwards, and the battle against Mettaton begins. After fighting and defeating Mettaton, his deactivated body/corpse is left behind, and Alphys unlocks the door and evaluates the situation. The protagonist continues into a horizontal hallway with the elevator to New Home at the end. Alphys follows closely behind, blocking the way back until the protagonist opens the elevator.
- The majority of enemies in the CORE are harder counterparts of enemies in the Ruins, which also appear in Hard Mode.
- Even though the CORE is technically part of Hotland, Mettaton is the only monster who can be encountered in both the CORE and other parts of Hotland.
- 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.
- The "smell of ozone" SAVE Point may be a reference to the VVVVVV room captioned "I Smell Ozone". Also, one of the instruments used in the track "CORE" is the synth named "PoiZone".
- Because this place is the only non-Hard Mode place to have Astigmatism, Whimsalot, and Final Froggit, and Reaper Bird is composed out of these three enemies, it is possible that Reaper Bird's home is set at the CORE.
- That's the CORE. The source of all power for the underground. It converts geothermal energy into magical electricity, by... Uhh, anyway, that's where we're going to go. In the CORE is an elevator directly to ASGORE's castle. - Alphys
- In the CORE is an elevator directly to ASGORE's castle. - Alphys
- I work at the CORE. The inside is a maze made of swappable parts... That means we can shuffle the layout at will. - Charles
- It makes sense why ASGORE took so long to hire a new Royal Scientist. After all, the old one... Dr. Gaster. What an act to follow! They say he created the CORE. However, his life... Was cut short. One day, he fell into his creation, and... - Gaster Follower 2
- (The air is filled with the smell of ozone... it fills you with determination.) - SAVE Point flavor text
- Papyrus: “THAT FURRY PERSON THROWS ICE ALL DAY. NOBODY KNOWS WHERE IT GOES.”
Undyne: “Hey, I know! Alphys told me... It goes to Hotland to cool down the CORE!”
Papyrus: “WOW, UNDYNE!! I DIDN’T KNOW YOU LIKED NERDY STUFF! JUST STRENGTH AND FIGHTING...”
Undyne: “Are you kidding!? Why wouldn’t I!? The brain’s the biggest muscle of all!!!”
- The phone call with Papyrus and Undyne in Ice Wolf’s room
- Instruments recognized in 65. CORE - Undertale Music Sample List. Retrieved September 3, 2018 from https://docs.google.com/document/d/1ZFusgT_6ROaajy9HkHrauSJY9vC7qP7oeEeYchPs2PE