The Chamber of Secrets, which is deep under the school (most likely under the lake), was home to an ancient Basilisk, intended to be used to purge the school of Muggle-born students.Salazar Slytherin, one of the founders of Hogwarts, built the Chamber before he left the school.
The Chamber is well-hidden and its entrance is in Moaning Myrtle's bathroom on the second floor, which leads down into a dark, slimy stone tunnel. There are many skeletons of small animals littering the floor and even a gigantic skin shed by the Basilisk. The tunnel leads to a solid wall, carved with two entwined serpents with emeralds for eyes. When Parseltongue is spoken they open into a long, dim corridor, lined with monumental statues of snakes, including two towering stone pillars with more carved serpents that brace the ceiling. A colossal statue of Salazar Slytherin, looking ancient and monkey-like, is at the centre. The Basilisk rested inside the statue and emerged from its mouth when the Heir of Slytherin, Tom Riddle, summoned it.In his second year at Hogwarts, Harry uses Parseltongue to open the chamber and destroys the diary containing the embodied memory of a 16-year-old Tom Riddle from his own days at Hogwarts. It is later revealed that the diary was a Horcrux. In Deathly Hallows, Ron and Hermione enter the Chamber. Ron opens the door (despite not speaking Parseltongue) by imitating sounds he heard Harry use to open Slytherin's locket. They find a basilisk fang to use to destroy the Horcrux made from Helga Hufflepuff's cup.
Moaning Myrtle's bathroom contains the entrance to the Chamber of Secrets. The entrance is a sink with a snake scratched onto the tap, opened by speaking Parseltongue. This causes the sink to open into a pipe large enough for a person to slide down it. At the bottom of this chute is a tunnel leading to the Chamber of Secrets. When Tom Riddle opened the Chamber, Myrtle was sulking in a stall. When she heard him, she opened the door, saw the Basilisk, and died immediately, becoming a ghost. Her bathroom remains operational, but is rarely used by students because of Myrtle's disagreeable presence and her habit of flooding it when she is distraught.