Waterfall is the third area the protagonist encounters in the Underground. The area is accessible after the protagonist defeats Papyrus in a fight. The area follows Snowdin Forest and Snowdin Town, and precedes Hotland.
- Napstablook, Undyne, and Mettaton have houses in Waterfall.
- The ancient glyphs on the walls describe The War of Humans and Monsters as well as the prophecy surrounding the Delta Rune.
- The protagonist encounters Mad Dummy and Monster Kid.
- Undyne attacks the protagonist on several occasions and is the last monster the protagonist battles in Waterfall.
- The protagonist finds the Old Tutu and Ballet Shoes behind a waterfall and in tall grass, respectively. Gerson sells the Cloudy Glasses and Torn Notebook.
- Temmie Village is hidden within Waterfall.
- A memory from the first human plays after the protagonist falls into the Garbage Dump.
Upon entering Waterfall, the protagonist passes Sans's sentry station, where Sans offers to take them to Grillby's, if they did not kill Papyrus. Proceeding through Waterfall, Monster Kid talks to the protagonist and expresses enthusiasm to see Undyne in action. Monster Kid unknowingly saves the protagonist from Undyne on a couple of occasions.
As they progress through puzzles involving bridge seeds, the protagonist speaks to Papyrus and Onionsan. During one encounter with Undyne, Undyne cuts the bridge that the protagonist is standing on, and the protagonist falls into the Garbage Dump. Before leaving the Garbage Dump, Mad Dummy fights the protagonist, and Napstablook invites the protagonist to their house.
After passing through rooms filled with Echo Flowers and mazes with lit paths, the protagonist fights Undyne. This battle leads to a chase scene into Hotland.
On a Genocide Route, the protagonist encounters Monster Kid and Undyne's spear attacks, but the puzzles are solved. Mad Dummy becomes corporeal and transforms into Glad Dummy. Every Temmie has left Temmie Village except for the Tem Shop shopkeeper and the Temmie that looks out from the wall. After the protagonist kills Undyne, she returns as Undyne the Undying. If there are still monsters left right before fighting Undyne, "Strongly felt <amount> left. Shouldn't proceed yet." will appear.
Ancient glyphs adorn walls throughout Waterfall. The color palette of this region is mainly purple and blue with the occasional black abyss. Mushrooms, Echo Flowers, and gems within the walls glow bright blue. True to its namesake, pools of water often provide the atmosphere in Waterfall, and the ceiling drips with water in some areas.
Waterfall Entrance to Initial Sea-Grass
In the first room of Waterfall, the protagonist speaks to Monster Kid, Sans, and the Echo Flower Explainer. A small waterfall flows through this room, and an Echo Flower and Sans's sentry station are against the north wall. Between Sans's sentry station and the Echo Flower is a SAVE point. Snow covers Sans's sentry station and his station blocks access to a wooden door. From this room, the protagonist can go with Sans to Grillby's via a shortcut through the right wall.
After progressing right, the protagonist encounters a puzzle that involves dodging rocks that are going down the waterfall as well as a box. If a rock pushes the protagonist over the edge of the waterfall, they land on a wooden bridge that has an Echo Flower on the rightmost end. Backtracking to the left allows the protagonist to attempt to cross the waterfall again or access the hidden room behind the waterfall. The Old Tutu resides in this hidden room.
To the right of the waterfall room, a room with a patch of tall grass provides a cutscene of Papyrus and Undyne. Papyrus stands on the lighter side of the room while Undyne stands in the shadows. Undyne nearly attacks the protagonist but stops when she remembers that the sea-grass is used for scientific study.
A few red gems and blue gems are in this room, along with three stalagmites near Undyne. After the protagonist exits the tall grass on the right, Monster Kid appears from the grass and celebrates seeing Undyne.
Initial Sea-Grass to The Wishing Room
Progressing right, the protagonist encounters Bridge Seed puzzles. The first room serves as a tutorial of Bridge Seed mechanics, and the protagonist crosses the waterfall in the room after lining up four Bridge Seeds. Using the right exit of the room, the protagonist enters a larger room with another Bridge Seed puzzle within it.
After taking a few steps forward, the protagonist encounters Aaron. This encounter is scripted.
Along with four Bridge Seeds, this room contains four glowing mushrooms, a sign in the middle of a waterlogged area, a Bell Blossom that retrieves the seeds, and a sign describing the Bell Blossom. The sign in the bottom left of this room reads "Congratulations! You failed the puzzle!" The protagonist can solve the Bridge Seed puzzle multiple ways.
When the protagonist approaches the top left exit of the Bridge Seed room, Papyrus calls and inquires what the protagonist is wearing. They will have a choice of whether to tell him the truth or lie to him.
The Wishing Room to Ferry Dock
The next room is the wishing room which contains many Echo Flowers and blue, glowing gems on the ceiling. As the protagonist progresses right, a telescope pointed at the ceiling offers a clue to check the northern wall to find the exit fo the room.
The northern exit leads to a room with a wooden bridge and dark water in which many Water Sausages grow. On the north wall are many ancient glyphs. At the end of the wooden dock, the protagonist steps on a small raft that drifts to the right.
Undyne's first spear barrage occurs in the next room. The protagonist flees from Undyne's spears along a wooden dock surrounded by black water. Undyne stands to the north of the protagonist and directly behind a row of rock pillars. The protagonist escapes Undyne by hiding in tall sea-grass.
Ferry Dock to Onionsan
After leaving this room through the right exit, the protagonist reaches the third SAVE point. Also in this room is an Echo Flower, a mouse hole, and a piece of cheese. The Echo Flower, which is to the right of the mouse hole, squeaks when the protagonist interacts with it.
Continuing right, the protagonist exits the room and sees Sans standing next to a telescope. To his right is another dimensional box. Sans attempts to prank the protagonist with the red-eye telescope. Exiting through the wooden door on the north wall of this room leads to another small room with the Nice Cream Guy in it. There is also a Punch Card box, a blue, glowing mushroom, and defiled ancient glyphs in this room.
If the protagonist backtracks from this room and travels right from Sans's telescope, they encounter "Loren" and, continuing right, the small bird. The protagonist cannot use the small bird yet, as they are on opposite sides of a chasm of bright blue water.
The room with "Loren" in it spans a large area horizontally and vertically. The color palette reverses in this room, and the water appears bright blue while the land appears black. As the protagonist traverses this room, they encounter a few Echo Flowers and can also find the Ballet Shoes in a tuft of tall grass.
Before the protagonist crosses a wooden bridge, one of four possible dialogs can happen, depending on what the player chose to tell Papyrus in the second Bridge Seed puzzle room. If they told him the truth, Papyrus calls them again and explains that he told Undyne exactly what the protagonist told him because he assumed the protagonist would change their clothes after being asked such a suspicious question. If they instead chose to lie, Papyrus will give one of two dialogs where he said he chose to lie about what they were wearing so they wouldn't get into trouble, accidentally telling Undyne what they actually are wearing. If the protagonist has changed their defensive item, Papyrus will say he made up what they were wearing, accidentally guessing what they changed into.
Continuing, the protagonist enters a long, horizontal room with a narrow black path leading to the right. As they progress down this path, Onionsan appears and talks to them. The protagonist then leaves through the right exit and encounters Shyren in the next room.
Onionsan to Garbage Dump
This room has two small, white waterfalls that flow from the top to the bottom of the screen and always stay in the foreground. To the north of this room, the piano puzzle awaits the protagonist. The protagonist opens the door to the Artifact Room by playing the melody that echoes through the area.
The Artifact Room itself contains a red orb on a pedestal and glyphs that cannot be read because the protagonist cannot stand directly in front of them.
Backtracking and heading to the right from the room with Shyren's battle, the protagonist encounters more ancient glyphs that inform them of Boss Monsters. Continuing to the right, they see an ancient statue that has water dripping on it from the ceiling.
Exiting this room through the right exit leads the protagonist to a room with a white waterfall in the foreground, a bucket of umbrellas, and a sign encouraging them to take an umbrella. Progressing right, they walk through a waterlogged room that has many waterfalls and reflective pools within it. Monster Kid walks alongside the protagonist for the duration of this room.
After the duo reaches the end of this room, they enter a two-dimensional room with New Home off in the distance. The game returns to an overworld scheme after the protagonist exits this room to the right.
After Monster Kid helps the protagonist climb onto a ledge, the protagonist progresses to the next room without them after Monster Kid gives them the go-ahead.
The next SAVE point is in the room immediately to the right of the ledge. There are also more ancient glyphs on the walls that detail The War of Humans and Monsters. Exiting this room to the right, the protagonist walks on another wooden bridge where Undyne antagonizes them. After reaching a dead end of the bridge, the protagonist falls into the garbage dump after Undyne slices the bridge in two.
Garbage Dump to Undyne's Crag
Mad Dummy is fought. Napstablook invites the protagonist to their house.
Small bird may be approached. Napstablook may be spoken with at his house or his snail farm, and offers a gambling opportunity at the latter. Mettaton's house is next door. Gerson may be bartered with, and the river person is present if the protagonist has reached Hotland. A dimensional box is present. Mazes involving lighted paths must be solved, and the Temmie Village may be found. Undyne attacks, but is inadvertently stopped by Monster Kid. Monster Kid then falls from a bridge and is saved by Undyne or the protagonist. Undyne speaks to the protagonist.
Undyne must be fought. The protagonist may then proceed into Hotland.
- One of Alphys's cameras can be found at the center of the bottom of the waterfall where rocks fall.
- Between the save point with the crystallised cheese and the room with Sans next to the telescope it is possible (random chance linked to the "fun" value) that an additional T-junction hallway will appear, room 268. The junction leads to room 269, which contains a figure many believe to be W. D. Gaster.
- Directly before the third SAVE point, the protagonist can backtrack through the tall grass area to the beginning of the room. Once they return to the end of this room, an NPC offers a ferry ride in which the protagonist can gain 6G.
- In the room with the Echo Flowers right before the bridge where you choose whether Monster Kid falls, if the player reaches the end of the room but not go into the bridge room, and instead, backtracks, you can see Flowey going back into the ground next to an Echo Flower. If the player then interacts with the Echo Flower, you'll instead hear Flowey disguising himself with Toriel's voice, with the dialogue varying depending on whether you killed her or not. And if it's interacted on a Genocide Playthrough, the Echo Flower still says "It's strangely silent", only now, it's in Toriel's voice.
- There is a glitch in the two-dimensional room that overlooks New Home. If the player holds the up, down, and right directional movement keys simultaneously, the protagonist moves at a faster pace.
- Waterfall resembles the Land of Wind and Shade from Homestuck. The Waterfall leitmotif originates from the song Doctor, which is the Land of Wind and Shade's theme.
- ↑ "Man, are you unlucky. If you were standing just a LITTLE bit to the left...! Yo, don't worry! I'm sure we'll see her again!" - Monster Kid
- ↑ "Eureka. Eureka! EUREKA! Human. That moment of unbridled emotion. It allowed me to finally fuse with my body! I'm fully corporeal now! My lifelong dream, realized!" - Glad Dummy
- ↑ "I swore I saw something... Behind that rushing water..." - Echo Flower Narration
- ↑ "This is the room where I first spotted you. You have NO idea how many spears I wanted to throw!!! But, um, that sea- grass is under scientific protection." - Undyne
- ↑ "I just wasn't ready for the responsibility." - Echo Flower Narration
- ↑ "NYEH HEH HEH HEH!! SO... WHAT ARE YOU WEARING...? I'M... ASKING FOR A FRIEND." - Papyrus
- ↑ "THE WISHING ROOM. DO YOU HAVE A WISH...? NYEH HEH HEH!! I HAVE ONE!!! I WISH I WAS TALKING TO MY COOL FRIEND!!! LOOK!!! IT'S COMING TRUE! WHAT WILL YOU WISH FOR?" - Papyrus
- ↑ "The ancient glyphs have been painted over with a list of 21 different flavors." - Narrator
- ↑ "There's normally a pair of old shoes in that seagrass. Strangely, they're made for someone without fins or claws. What kind of monster is like that...?" - Undyne
- ↑ "BECAUSE I KNEW, OF COURSE...AFTER SUCH A SUSPICIOUS QUESTION...YOU WOULD OBVIOUSLY CHANGE YOUR CLOTHES! YOU'RE SUCH A SMART COOKIE! THIS WAY YOU'RE SAFE AND I DIDN'T LIE!!! NO BETRAYAL ANYWHERE!!! BEING FRIENDS WITH EVERYONE IS EASY!!!" - Papyrus
- ↑ "I KNEW WHEN YOU SAID: "I AM NOT WEARING A (ITEM)" IT WAS REALLY A SECRET CODE! YOU REALLY MEANT..."I ACTUALLY AM WEARING A (ITEM)!" YOU WERE TRYING TO PROTECT YOURSELF...WHILE MAKING IT SO I DIDN'T HAVE TO LIE! I PICKED UP ON THIS, AND FOLLOWED YOUR PLAN. IN FACT I TOOK IT ONE STEP FURTHER! I TOLD HER YOU WERE PROBABLY...WEARING A (ITEM) OF COURSE, YOU WOULD NEVER WEAR THAT. BUT THAT'S THE POINT! SHE WON'T RECOGNIZE YOU NOW! AND I DIDN'T HAVE TO BETRAY EITHER OF YOU! SINCE I JUST TOLD HER WHAT YOU SAID!" - Papyrus
- ↑ "PAPYRUS WOULD NEVER BETRAY YOU! YOU SAID YOU WERE NOT WEARING A (ITEM) SO OF COURSE I ACTUALLY TOLD HER...YOU WERE INDEED WEARING A (ITEM) BUT SINCE YOU AREN'T WEARING A (ITEM) SHE SURELY WON'T ATTACK YOU! NOW YOU ARE SAFE AND SOUND. WOWIE... THIS IS HARD. I JUST WANT TO BE EVERYBODY'S FRIEND!" - Papyrus
- ↑ "I TOLD HER YOU WERE NOT WEARING A (PREVIOUS ITEM) EVEN THOUGH YOU TOLD ME YOU WERE! INSTEAD, I MADE SOMETHING UP! I TOLD HER YOU WERE WEARING...A (CURRENT ITEM) IT PAINED ME TO TELL SUCH A BOLDFACED LIE. I KNOW YOU WOULD NEVER EVER WEAR A (CURRENT ITEM) BUT YOUR SAFETY IS MORE IMPORTANT THAN FASHION." - Papyrus
- ↑ "A haunting song echoes down the corridor... Won't you play along?" - Glyph in the piano puzzle room
- ↑ "There is only one exception. The SOUL of a special species of monster called a 'Boss Monster.' A Boss Monster's SOUL is strong enough to persist after death... If only for a few moments." - Ancient glyph
- ↑ "Yo, you go on ahead. Don't worry about me. I always find a way to get through!" - Monster Kid