The First floor girls' lavatory, more commonly known as Moaning Myrtle's Bathroom is situated on the first floor of Hogwarts Castle, just above the Great Hall. It has been out of order ever since a student named Myrtle Warren was killed there. Myrtle has haunted the bathroom ever since the Ministry of Magic stopped her from haunting Olive Hornby who had been teasing Myrtle just before her death, leading it to be a place most students do not want to enter. This bathroom holds the entrance to the Chamber of Secrets. To get in the Chamber, a person needs to say "open" in Parseltongue. This will cause the tap with a snake on the side (a tap which incidentally, never worked) to move into the floor, revealing a tunnel which leads downwards into the Chamber.
Founding of Hogwarts
Hogwarts Castle and this bathroom were built by Godric Gryffindor, Rowena Ravenclaw, Helga Hufflepuff and Salazar Slytherin. It was built inside a turret. According to an ancient legend, Salazar Slytherin was responsible for the construction of the Chamber of Secrets beneath Hogwarts dungeons. The Chamber contained a Basilisk which could only be controlled by Slytherin's true heir, and use it to purge the school of all muggle-born students. Before being forced out of Hogwarts by the other founders, Slytherin hid the entrance to the Chamber. Several centuries later in the 1700s, a direct of descendant of Slytherin, Corvinus Gaunt, further concealed the passageway behind bewitched sinks that could be only opened by someone who spoke Parseltongue. For centuries, the Castle was thoroughly searched and no such Chamber was found until one of Slytherin's descendents set foot in the Castle in the late 1930s.
Chamber of Secrets openings (1943)
In 1938, Tom Riddle (best known by the alias "Lord Voldemort") started his education at Hogwarts School of Witchcraft and Wizardry. In his fifth year, he learned of his Slytherin ancestry and discovered the existence and entrance to the Chamber of Secrets and tamed the Basilisk which dwelt within. As the Heir of Slytherin through his mother's family, Tom was able to open the Chamber of Secrets and started purging the school "of all those who were unworthy of study magic" - in Riddle and Slytherin's eyes those were Muggle-born students. Many students were attacked in 1943. The attacks eventually stopped with the murder of a student named Myrtle Warren inside this bathroom.
After her death, Myrtle's ghost (or Moaning Myrtle, as the students usually call her) haunted Olive Hornby, the fellow student who caused her to be in the bathroom that evening. When the Ministry of Magic refrained her from doing so, Moaning Myrtle returned to the bathroom, haunting it from that day on.
Chamber of Secrets openings (1992-1993)
Hermione Granger brewed Polyjuice Potion there in her Second year, in 1992. She, Harry Potter and Ron Weasley used it as part of their unsuccessful plan to question Draco Malfoy about the opening of the Chamber of Secrets. Later in the school year, Harry Potter and Ron Weasley discovered where the entrance to the Chamber was and they, along with Gilderoy Lockhart, then-Defence Against the Dark Arts Professor, entered the Chamber of Secrets via this bathroom.
Behind the scenes
- The location of Moaning Myrtle's Bathroom has been described inconsistently.
- Hermione initially mentions it as being on the first floor, but later Harry is said to run up two floors from the Entrance Hall.
- Two years later, Harry also describes it as being on the second floor. 
- On Pottermore , it was simply called "Moaning Myrtle's Bathroom".
- In Harry Potter and the Order of the Phoenix (video game), and Harry Potter and the Half-Blood Prince (video game), Myrtle's Bathroom is located on the first floor, but across the Stone Bridge.
- In the film adaptation of Harry Potter and the Chamber of Secrets, all the sinks in the bathroom are arranged in a column, which separates before each sink descends into the ground to create the passage into the Chamber of Secrets.
- In the video game adaptation of Harry Potter and the Half-Blood Prince this lavatory is replaced by a male bathroom, with different sinks and urinals instead of stalls. In the movie adaptation, however it can be seen that the corridor outside the male bathroom is different from the corridor outside Myrtle's bathroom.
- Harry Potter and the Philosopher's Stone (video game) (GBC version only)  (Entrance only)
- Harry Potter and the Chamber of Secrets (First appearance)
- Harry Potter and the Chamber of Secrets (film)
- Harry Potter and the Chamber of Secrets (video game)
- Harry Potter and the Goblet of Fire (Mentioned only)
- Harry Potter and the Order of the Phoenix (video game)
- Harry Potter and the Deathly Hallows (Mentioned only)
- LEGO Harry Potter: Years 1-4
- Harry Potter Trading Card Game
- Harry Potter: The Character Vault
Notes and references
- ↑ 1.0 1.1 Pottermore - From the story: Moaning Myrtle's Bathroom
- ↑ See this image.
- ↑ Pottermore - Writing by JK Rowling: Chamber of Secrets chamber-of-secrets Writing by JK Rowling: Chamber of Secrets
- ↑ Harry Potter and the Chamber of Secrets, Chapter 8 "She (Moaning Myrtle) haunts one of the toilets in the girls’ bathroom on the first floor,” said Hermione.
- ↑ Harry Potter and the Chamber of Secrets, Chapter 8 “This way,” he shouted, and he began to run, up the stairs, into the entrance hall. It was no good hoping to hear anything here, the babble of talk from the Halloween feast was echoing out of the Great Hall. Harry sprinted up the marble staircase to the first floor, Ron and Hermione clattering behind him. “It’s going to kill someone!” he shouted, and ignoring Ron’s and Hermione’s bewildered faces, he ran up the next flight of steps three at a time, trying to listen over his own pounding footsteps — Harry hurtled around the whole of the second floor, Ron and Hermione panting behind him, not stopping until they turned a corner into the last, deserted passage.
- ↑ Harry Potter and the Goblet of Fire, Chapter 22 “I suppose there’s always Moaning Myrtle,” he said gloomily, referring to the ghost who haunted the girls’ toilets on the second floor.
- ↑ Despite it's locked, there is a doorway on the second floor with pavement around it and is flooded. The same doorway serves as the entrance to Myrtle's Bathroom in the next instalment.