- This area is the point of no return for the True Pacifist Route. Upon entering the True Lab, the True Pacifist Route can no longer be aborted, and most of the Underground becomes inaccessible until the epilogue.
- The information that can be gathered in this location provides an insight on Alphys's personality and some of the causes of her anxiety, insecurity and lack of self-esteem, as well as some detail about how she and Mettaton began to grow more distant.
- The screens on the walls reveal how Flowey came to be.
- The amalgamates are introduced, along with the details about Alphys's experiments with human SOULs in an attempt to find a way to destroy the Barrier.
- The concept of determination is explained in greater detail, including its effects on both humans and monsters and how it was extracted and used by Alphys in her experiments.
- It is also revealed that SOULs cannot be artificially created, contrary to what Alphys told King Asgore in order to get her job as a royal scientist.
True Pacifist Route
After the protagonist has completed the date with Alphys by giving her Undyne's Letter, Papyrus calls the protagonist and strongly recommends that they meet Alphys at her lab. When the protagonist arrives at the Lab, there is a note in front of the previously locked door, which now opens upon approaching.
After entering the door, the protagonist descends in an elevator that malfunctions and crashes. Once the protagonist enters and opens the elevator door, they cannot return to the rest of the Underground as they are locked in the True Lab. The protagonist traverses the True Laboratory where they find several Lab Entries, Amalgamates, and four keys which give access to the power supply to restart the elevator.
In the True Lab, the protagonist learns about how Alphys experimented with determination, SOULs, fallen monsters, and Golden Flowers. While working with Determination, Alphys resurrected fallen monsters such as Shyren's sister and Snowdrake's Mother, who as a result soon melted into Amalgamates. The protagonist also discovers that Alphys attempted to create a soulless vessel from a Golden Flower; the vessel is heavily implied to be Flowey. Alphys's lab entries provide backstory on her experimentation and are found throughout the True Lab.
While exploring the Lab, the protagonist encounters a strange machine. This machine is the DT Extraction Machine, which Alphys used to extract determination from the six previously captured human SOULs. The protagonist also finds several VHS tapes that lack video motion but retains its audio. They document conversation between Toriel and Asgore, and Asriel and the first human. These tapes imply that the fallen human possessed darker tendencies and that they attempted suicide as part of a plan to combine their SOUL with Asriel's to pass through the barrier. The VHS TV in this room resembles the TV that projects Photoshop Flowey's face.
The protagonist then restores power to the elevator. Moments later, Amalgamates surround them. Alphys rushes into the room, saves the protagonist, and states that Amalgamates can get violent when not fed. After the Amalgamates leave the room, Alphys thanks the protagonist for the help and mentions that it is not the Amalgamates which would have made her "disappear" but instead something cowardly which she might do. This cowardly act may be suicide considering that some neutral endings tell of Alphys disappearing.
As the protagonist makes their way to the elevator, the exit cuts off, and a voice is heard congratulating the protagonist. The elevator then leaves the protagonist in New Home, with the elevator down to the CORE sealed off with vines so that the protagonist cannot leave. This indicates that the requirements have been met for the True Pacifist Ending.
Genocide and Near-Genocide Neutral Route
Upon defeat, Undyne the Undying informs the protagonist that Alphys has evacuated the majority of the remaining monsters someplace safe, heavily implied to be the True Lab. In the near-genocide ending, Alphys talks for most of the endgame phone call. She says that, despite having to reveal the truth about herself to everyone, people still treat her like a hero.
In the Genocide ending, the evacuation was pointless, as the first human destroys the entire world and ends the timeline.
In contrast to the bright, sterile color scheme of the Lab, the True Lab is grimy, dark, and fog-filled. It has a dark green celeste color scheme and an unsettling atmosphere. Assorted objects fill the area, including beds, sinks, refrigerators, wall fans, mirrors, and Golden Flowers. There is a vending machine that dispenses Popato Chisps near the power room door.
Entry Elevator to Power Room Door
Entries 1, 2, 3, and 5 are on the walls. The room with the power room door, contains 2 fake plants, the Popato Chisps machine, a third door, and a partially readable note from Alphys stating the door to the second elevator must be entered. The power room door may only be opened after all keys are used, and the third door may only be opened after the red key is used.
Power Room Door to Red Key Slot
Entries 6 and 9 are present. A room contains several sticky operating tables and 3 sinks. One of the sinks offers the red key and Memoryhead. Nearby is a slot for the red key.
Third Door to Dormitory
Entires 12, 14, and 13 are present. The bedroom contains a dog food bowl, 4 fake plants, a broken clock, and 9 beds. The bottom left bed can be slept in, and the middle right bed contains the yellow key.
Dormitory to Fan Room
Entry 11 is present. The area is initially clouded by fog. A switch may be pressed to turn on the fans and blow the fog away. It forms Endogeny as a result who the protagonist fights.
Dormitory to DT Extractor Room
Entries 4, 15, and 16 are present. A hallway with a bathtub contains an amalgamate who disappears upon opening the curtain, leaving behind the green key. To the west is a large room containing the DT extractor, which is inactive. North of this room is a second hall leading to a room with a TV and some VHS tapes, the yellow key slot, and a note suggesting the location of the yellow key. Next to the DT extractor is Lemon Bread, disguised as a SAVE Point.
DT Extractor Room to Green Key Slot
Entries 19, 20, and 21 are present. There is a room with several fridges (one of which is shaking) and Snowdrake's mother, who can only be found if the fans have been turned on in the fan room. Otherwise, the room is shrouded in a thick fog which prevents the fridges from being seen, and their descriptions upon inspection are mere conjectures. The fridges are either empty or contain samples of some sort. Snowdrake's mother drops the blue key when spared. In the adjoining room is the slot for the green key.
Dormitory to Blue Key Slot
Entries 7 and 10 are present. Adjoining the dormitory is a room with mirrors, Golden Flowers, and Reaper Bird. After that is a room with the blue key slot and a hint as to the blue key's location.
Power Room to Power Room door
An elevator is passed through. Entries 8 and 18 are readable after the power is turned on. The Power Room contains a generator which powers up the elevator up to New Home. This generator has a red heart-shaped figure on its front.
Lab Entries can be found on the numerous panels can be seen throughout the lab. The entries are numbered from 1 to 21, but there is no 17th entry. In the game data, there are two 17th lab entries which cannot be accessed in normal gameplay; one of them is completely in Wingdings and presumed to have been written by W. D. Gaster. Most of the entries contain extensive information about Alphys's experimentation concerning SOULs, Determination, monsters, and Flowey.
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The five VHS tapes can be found in the room with the TV, which is north of the room with the DT Extraction Machine. They contain recordings of Asriel, his family and the Fallen Child interacting. However, these do not have any visual content.
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- There is a known glitch in which the Blue Key cannot be seen after being dropped but can still be picked up.
- The teacup mentioned in Entry Number 20 can be found in Alphys's lab, sitting on her desk.
- The DT Extraction Machine also heavily resembles a Cervine (Deer) skull, the same way Sans's Gaster Blasters resemble Canid (Dog) skulls, and Asriel's skull during the final battle resembles a Capra (Goat) skull.
- In the room with the beds, laying down on the bottom left one for a short while will cause an amalgamate to appear and extend out an arm to pull up the cover over the protagonist, then pat their head before disappearing.
- If the protagonist enters the refrigerator room without turning on the ventilation first, the fog will change the protagonist’s interactions with the environment to vague assumptions. The refrigerators will be described as "It's some sort of cold, rectangular object." Other observations are:
- "There's something here... It feels like it's breathing."
- "There’s something here... It feels like a tree, ripe with delicious fruit."
- "There's something here... It feels warm, and moist."
- "There’s something here... Something in the shape of a man."
- The screens for Entry Numbers 8 and 18 display a red smile instead of green lines like the other entries, possibly because they tell of how Alphys created Flowey, and Flowey disguises his dark intentions with an innocent smile.
- If the protagonist has Popato Chisps in their inventory, Alphys says in the power room Amalgamates probably smelled the chisps in their inventory.
- ↑ Exiled Queen Ending (Undyne left alive) and the Mettaton Ending
- ↑ W.D Gaster: Undertale's Last Secret – Mistress Spitfire on Steam Community