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 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.

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, 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.

He was standing at the end of a very long, dimly lit chamber. Towering stone pillars entwined with more carved serpents rose to support a ceiling lost in darkness., casting long, black shadows throught he odd, greenish gloom that filled the place.

As he drew level with the last pair of pillars, a statue high as the Chamber itself loomed into view, standing against the back wall...it was ancient and monkeyish, with a long thin beard that fell almost to the bottom of the wizard's sweeping stone robes, where two enormous gray feet stood on the smooth Chamber floor.

* could it have any links with the Horcruxes, which have been stricktly forbidden topic at Hogwarts?
* why Salazar Slytherin took so much pain to build a hidden chamber?

After the killing of the basilisk, the Chamber might have been explored freely and, to open the hidden entrance, there was a Parseltongue available for Dumbledore - Harry. The headmaster has never suggested returning to the Chamber. On the other side, he didn't close it pernamently. A plan? A lack of curiosity? A negligence?

I understand that the Chamber was sealed by a rock fall where the ceiling in the passage collapsed. That was when Lockhart's memory charm backfired due to Ron's damaged wand.

Even a parseltongue might not be able to proceed further into the Chamber if the way has been blocked. So far, there has been at least one further mention of this rock fall - Fred and George mentioned later on in POA that one of the passages on the marauders map was sealed by a rock fall, possibly that very rock fall. So no wonder DD didn't investigate the Chamber further.