First floor girls' lavatory

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 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. Slytherin hid the entrance to the Chamber behind the sinks in this lavatory, and bewitched the sink so that it 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 inside this bathroom.

Myrtle's ghost
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 indefinitely.

Chamber of Secrets openings (1992-1993)
On Hallowe'en night, 1992, Tom Riddle's Diary (one of Voldemort's Horcruxes), possessed student Ginny Weasley and forced her to open the Chamber of Secrets (which had been closed since 1943).

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

 * 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.

Appearances

 * Harry Potter and the Philosopher's Stone (video game)
 * Harry Potter and the Chamber of Secrets
 * Harry Potter and the Chamber of Secrets (film)
 * Harry Potter and the Chamber of Secrets (video game)
 * Harry Potter and the Goblet of Fire 
 * Harry Potter and the Order of the Phoenix (video game)
 * Harry Potter and the Deathly Hallows
 * LEGO Harry Potter: Years 1-4
 * Harry Potter Trading Card Game
 * Pottermore