- A package of Instant Noodles can be obtained from Alphys's fridge.
- Alphys and Mettaton make their first appearances here, with the latter subjecting the protagonist to a quiz show.
- The True Lab, initially disguised as a bathroom, is found here, containing the results of Alphys's ill-fated experimentation with SOULs and Determination.
The Lab is where the protagonist first encounters Alphys after escaping from Undyne. Initially, the lights are turned off, and the protagonist cannot inspect anything because "It's too dark to see near the walls." Here, Mettaton subjects them to a quiz show. After the quiz show, the protagonist has their Cell Phone upgraded. They then exit the Lab and proceed on their way towards the CORE. The doors to the Lab lock after the fight against Mettaton EX.
In the Alphys Ending of the Neutral Route, achieved if the protagonist aborts the Genocide Route before reaching Mettaton NEO, it is implied that when Alphys evacuated the monsters, she brought them to the True Lab. Alphys tells them that she knew she had to do something about the protagonist's actions, even if it meant that the monsters would learn the truth about her.
True Pacifist Route
The protagonist delivers a letter from Undyne to Alphys, who reluctantly opens it and mistakes the protagonist as the writer of the letter. She then goes on a "date" with them, with Undyne and Papyrus making appearances. Papyrus sends her back to the Lab and calls the protagonist to recommend they go there as well.
Checking the bin between the computer and screen reveals that someone sent an intimidating message to Alphys, simply saying "I KNOW WHAT YOU DID," written in a strange handwriting. The "bathroom" is open, and near the doorway is a note from Alphys addressed to her friends, telling them that she needs to face her mistakes and that "the truth" is past the door. The protagonist enters the door, which is an elevator that takes them to the True Lab.
In the Genocide Route, the lights in the Lab are already on, and Alphys is not present. Instead, Mettaton appears and explains that Alphys has been evacuating everyone to a safe location. Mettaton leaves after the protagonist approaches him aggressively.
The Lab is a large white and red building with a pale green interior. There are two floors on the lab, the ground floor being Alphys's workplace, and the second floor being her room.
On the ground floor, there is a large screen where Alphys monitors the protagonist, a fridge with a supply of Instant Noodles, a messy desk with her computer, and a "bathroom" that leads to the True Lab. Alphys's obsessions and hobbies, including comics, anime figures, an ice cream machine, etc. litter her room, which is on the second floor On this floor, there are unopened letters from many monsters, presumably from the Amalgamates' families.
- In the Windows version of Undertale, the monitor follows the protagonist's movements, whereas the screen displays a static image in the macOS and Linux versions.
- It is possible to skip the entire scene with Alphys and Mettaton by interacting with the bag of dog food right when the scene begins. This has been dubbed the "Lab Skip."
- In Undertale v1.001, the hitbox of the bag of dog food was shrunk to prevent this from happening. However, because Toby Fox could not figure out how to replicate it, this did not fix it.
- Interestingly, calling Papyrus's Phone twice in the bottom floor of the lab now causes him to recall seeing Sans with said dog food bag. This may be an in-game reason to explain why the bag's hitbox has shrunk.
- The "Lab Skip" has been abandoned by the speedrunning community for the "Punch Card Exploit" since it is a much easier alternative to pull off.
- The bathroom sign and the switch around the entrance to the True Lab have been moved in Undertale v1.001.
- A rather amusing glitch before v1.001 can be pulled off by calling Papyrus and Undyne while on a conveyor. This causes the text and portrait to trail off the screen.
- Inspecting the bag of dog food yields a message describing it as "half-full" if no monsters have been killed. If any monster is killed, the message changes to "half-empty." A follow up message, "You just remembered something funny," appears if Doggo, Lesser Dog, and Greater Dog are killed and the protagonist has 21+ kills.
- "YOU'RE IN HER ROOM, UNINVITED? WOWIE... THAT'S EXTREMELY CREEPY." - Papyrus, phone call in room room_fire_lab2. Retrieved from https://pcy.ulyssis.be/undertale/calls
- "so far bugfixes include shrinking the hitbox of a bag of dog food" - Toby Fox. January 10, 2016. Twitter.
- "well it's at least harder to do now (?) i couldn't figure out how to do it so its probably ok if its just a speedrun thing" - Toby Fox. January 20, 2016. Twitter.