Hermione:
Professor, I was wondering if you could tell us about the Chamber of Secrets.
Professor Minerva McGonagall:
[seeing everyone's faces]
Very well. Well, you all know, of course, that Hogwarts was founded over a thousand years ago, by the four greatest witches and wizards of the age. Godric Gryffindor, Helga Hufflepuff, Rowena Ravenclaw, and Salazar Slytherin. Now three of the founders co-existed quite harmoniously. One did not.
Ron:
Three guesses who.
Professor Minerva McGonagall:
Salazar Slytherin wished to be more selective about the students admitted to Hogwarts. He believed magical learning should be kept within all magic families. In other words, pure bloods. Unable to sway the others, he decided to leave the school. Now according to legend, Slytherin had built a hidden chamber in this castle, known as the Chamber of Secrets. Though shortly before departing, he sealed it until that time when his own true heir returned to the school. The heir alone would be able to open the chamber and unleash the horror within, and by so doing, purge the school of all those who, in Slytherin's view, were unworthy to study magic.
Hermione:
Muggle-borns.
Professor Minerva McGonagall:
Well naturally the school has been searched many times. No such chamber has been found.