"It is a myth! It does not exist! There is not a shred of evidence that Slytherin ever built so much as a secret broom cupboard! I regret telling you such a foolish story!"
-- Professor Binns (CS9)
The Chamber of Secrets is a legendary secret room constructed by Salazar Slytherin to house a monster which would finish his “noble work.” This noble work would be accomplished when his true heir would open the Chamber and release the monster which lived within to rid the school of non-pureblood students.
The Chamber is well hidden. The entrance is behind a sink in Moaning Myrtle’s bathroom on the second floor. There is a tiny snake figure scratched into the tap and when someone orders “open up” in Parseltongue, the sink recedes and a pipe appears. Sliding down this pipe takes one to an underground passage, probably located under the lake. There are bones of small animals here and Harry and Ron discover the shed skin of the basilisk nearby when they venture down (CS16).
The Chamber itself is a huge temple-like room with pillars and carved serpents. There is a massive statue of Salazar Slytherin at the far end from which the basilisk comes when summoned. Harry battled and defeated the basilisk here on May 29, 1993, with the help of Fawkes the phoenix and the old school Sorting Hat.
According to the legend, Slytherin and Gryffindor argued over whether to allow Muggle-born students into Hogwarts. When Gryffindor apparently won, Slytherin left the school, but not before building the Chamber. It became a shrine to himself, a temple-like room decorated with snake carvings and a massive statue of himself. Over the centuries, many great witches and wizards have searched for the Chamber but only those who spoke Parseltongue could find it, most of them in the direct family line of Slytherin himself (Pm,CS16, DH31). Everyone else assumed that the Chamber was purely fiction (CS9).
As Hogwarts grew over the centuries, Muggle-style plumbing threatened the trapdoor entrance to the Chamber, and was protected by the timely intervention of Corvinus Gaunt, a Parselmouth descended from Slytherin, who made sure it could not be seen by anyone else (Pm). The entrance is behind a sink in a bathroom on the second floor. There is a tiny snake figure scratched into the tap and when someone orders "open up" in Parseltongue, the sink recedes and a pipe appears. Sliding down this pipe takes one to an underground passage, probably located under the lake. There are bones of small animals piled all over the floor, and this is where years later Harry and Ron discovered the giant shed skin of the Basilisk when they ventured down (CS16).
A thousand years after Slytherin built the Chamber, his heir, Tom Riddle, started at Hogwarts. Riddle spent several years discovering how to open the Chamber and when he did, he unleashed the monster on the school. The monster was a Basilisk, which Tom could control since it is a great serpent and spoke Parseltongue as well. There were a number of attacks on Muggle-born students, ending in the death of Myrtle in a second-floor bathroom.
Tom's plan backfired on him, however. When the girl died, the headmaster, Armando Dippet, decided to close the school. Riddle came from a Muggle orphanage and he would have had to live there if Hogwarts closed. So Riddle framed Hagrid for the crime of opening the Chamber. Hagrid was expelled and the school remained open (CS17).
After the spirit of Tom Riddle returned to Hogwarts through his Horcrux-laden diary, he released the Basilisk to attack Muggleborns in the school. Luckily no one died this time, but many were petrified by the stare of the evil creature. After Ginny Weasley was possessed by Tom Riddle and taken into the Chamber, Harry battled and defeated the Basilisk on May 29, 1993, with the help of Fawkes the phoenix and the old school Sorting Hat containing the Sword of Gryffindor (CS17).
During the Battle of Hogwarts, Ron managed to approximate the sound of the Parseltongue word for "open" and he and Hermione entered the Chamber to recover Basilisk fangs to be used to destroy Horcruxes (DH31).
Harry was able to hear the Basilisk and open the Chamber of Secrets although he was not a direct descendent of Salazar Slytherin. His ability as a Parselmouth came from Voldemort's accidental horcrux that attached itself to his forehead inside his lightning-shaped scar (DH35). That is also the reason the Sorting Hat considered putting him in Slytherin House, but Harry decided he'd rather be in Gryffindor because he disliked Draco so much (PS7).
"I always knew the chamber was there. I don't know what first gave me the idea; I just liked the thought that Slytherin had left something of himself behind."
-- J.K. Rowling (Sch2)