In an update on Pottermore, J.K. Rowling has given some further information about the history of the Chamber of Secrets. In it, she makes clear that the secret of the Chamber was passed down the generations of Slytherin descendants. They definitely knew the Chamber was there; they weren't just passing on rumours.
The existence of the Chamber was known to Slytherin’s descendants and those with whom they chose to share the information. Thus the rumour stayed alive through the centuries.
Some of those descendents did open the Chamber (in the sense of going inside it, not in the sense of releasing the Basilisk).
There is clear evidence that the Chamber was opened more than once between the death of Slytherin and the entrance of Tom Riddle in the twentieth century.
A specific line of Slytherin descendants - the Gaunts - were Parselmouths. They not only knew that the Basilisk existed but actually heard its snake-like voice moving around the castle. They also boasted freely about this.
Whispers that a monster lived in the depths of the castle were also prevalent for centuries. Again, this is because those who could hear and speak to it were not always as discreet as they might have been: the Gaunt family could not resist boasting of their knowledge. As nobody else could hear the creature sliding beneath floorboards or, latterly, through the plumbing, they did not have many believers, and none, until Riddle dared unleash the monster on the castle.
One of the Gaunts was involved in hiding the Chamber when the girls bathroom was built on top of the entrance.
The presence in school at the time of a student called Corvinus Gaunt – direct descendant of Slytherin, and antecedent of Tom Riddle – explains how the simple trapdoor was secretly protected, so that those who knew how could still access the entrance to the Chamber even after newfangled plumbing had been placed on top of it.
Given all this, why did it take until Riddle's day for the Chamber to be properly opened again?
Successive generations of Gaunts:
- Knew for sure about the existence of the Chamber and how to open it
- Could speak Parseltongue and control the Basilisk
- Were probably in many cases rabid blood purists who believed themselves to be superior to Muggle-borns
Successive generations of Gaunts seem to have had both the ability and the inclination to fully open the Chamber of Secrets and release the Basilisk. Why then did none of them actually do it?