i dont understand how the chamber could have been created w/o anyone knowing

Simple. Slytherin, one of the four founders, created it in the school while it was being built, and didn't tell anyone. He could have put repelling charms around the bathroom while he was making it, so he could do it without anyone finding out. Then, when it was finished, he made it so you have to be speak Parseltongue to get in--so if/when people DID find out and WERE able to enter the Chamber, they were typically Dark wizards (especially since most or all of them were descendants of Slytherin), so they didn't tell anyone, but kept the secret to themselves so more evil could be done from it.

Ok, Simple. Salazar Slytherin, one of the Four Founders of Hogwarts. Only wanted Pure-Bloods at Hogwarts. But the other 3 did not. As they thought that all Magic should be taught to Pure-Bloods and Known. So Slytherin built a Secret Chamber and inside the chamber was a Basilisk this Basilisk was suppose to get rid of all the non-Pure Bloods. But Once Salazar Slytherin Died, The Heir of Slytherin was the only person to open up the chamber. Thats the story..

The way most everything happens in HP: magic. Salazar Slytherin was a very powerful, and evil wizard who chose to leave a place of his own at Hogwarts for his heir. He built a chamber under the castle, and used his power of Parseltounge to make a powerful lock-- but not before he sealed a Basilisk inside. He probably used magic to join it to the pipes to form 'secret' passages through Hogwarts, to scare future students. How could he have done it without anyone knowing? The same answer as before. Magic. Solitude and Silence would have helped as well, for if he built the Chamber after the fight with the other founders, then the certainly would have left him alone. Though somehow, word must have leaked, because the legend lived on...

-Kel