Chamber of Secrets

"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

"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)

Atlas of Hogwarts: map of The Chamber of Secrets

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.

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. Over the centuries, many great witches and wizards have searched for the Chamber but none found it. Eventually, the whole thing was assumed to be purely fiction (CS9).

It was not, however. The reason no one could find the Chamber is that it required someone to speak in Parseltongue to open its entrance (CS16, DH31), and this was a power which only a very few have ever had. Slytherin spoke Parseltongue and a thousand years later, his heir, Tom Riddle, also had that ability. 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).

The Chamber of Secrets is well hidden. 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 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.

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).

Related links:
How did the Chamber of Secrets' entrance, constructed almost a thousand years ago, get hidden inside modern plumbing?