It’s obvious in the book that the professors and other wizards searched the school a couple of times, trying to find the Chamber of Secrets. Why didn’t they discover it? I mean, I’m sure they searched in Myrtle’s bathroom, especially because she died there.
They didn't speak Parseltongue
Successive headmasters and mistresses, not to mention a number of historians, searched the castle thoroughly many times over the centuries, each time concluding that the chamber was a myth. The reason for their failure was simple: none of them was a Parselmouth.
Pottermore - Chamber of Secrets
Hogwarts was good at hiding secrets.
Dumbledore only found the Room of Requirement after being headmaster at Hogwarts for a long time:
“Oh I would never dream of assuming I know all Hogwarts’ secrets, Igor,” said Dumbledore amicably. “Only this morning, for instance, I took a wrong turning on the way to the bathroom and found myself in a beautifully proportioned room I have never seen before, containing a really rather magnificent collection of chamber pots. When I went back to investigate more closely, I discovered that the room had vanished. But I must keep an eye out for it.
The Yule Ball - Goblet of Fire
Also, the Heir of Slytherin may have been a requirement. The message on the wall was:
"The Chamber of Secrets has been opened. Enemies of the heir, beware."
The Duelling Club - Chamber Of Secrets
And when Professor Binns explains the finer details of the Chamber, he tells us that:
“Slytherin, according to the legend, sealed the Chamber of Secrets so that none would be able to open it until his own true heir arrived at the school. The heir alone would be able to unseal the Chamber of Secrets, unleash the horror within, and use it to purge the school of all who were unworthy to study magic.”
The Writing On The Wall - Chamber Of Secrets
It may have been necessary for the Heir to be at the school for the Chamber to be accessible, so searching when an heir wasn't there could have been fruitless.
The heir was present the second time through the proxy of Riddle's diary, and the only other known opening of the Chamber was when Riddle himself was there. I'm not sure if Voldemort's family went to Hogwarts, and they don't seem intelligent enough to discover the Chamber. I'm just speculating, but the presence of Slytherin's heir may have been needed for the Chamber to even be accessible.
This was briefly alluded to in one of the other answers, but it was evidently magically protected to make it difficult or impossible for anyone other than Slytherin's Heir to find it. (After all, that was the entire point of the Chamber in the first place). For example, given its absence from the Maurauder's Map, it may have been unplottable (although it's admittedly somewhat unclear if the map only showed the parts of the Castle that the Maurauders were actually aware of; apparently, it didn't include the Room of Requirements, either).
When Hermione asked Professor Binns about it, they had the following exchange:
"[The horror within the Chamber of Secrets]... is believed to be some sort of monster, which the Heir of Slytherin alone can control," said Professor Binns in his dry, reedy voice.
The class exchanged nervous looks.
"I tell you, the thing does not exist," said Professor Binns, shuffling his notes. "There is no Chamber and no monster."
"But, sir," said Seamus Finnigan, "if the Chamber can only be opened by Slytherin's true heir, no one else would be able to find it, would they?"
"Nonsense, O'Flaherty," said Professor Binns in an aggravated tone. "If a long succession of Hogwarts headmasters and headmistresses haven't found the thing -"
"But, Professor," piped up Parvati Patil, "you'd probably have to use Dark Magic to open it -"
"Just because a wizard doesn't use Dark Magic doesn't mean he can't, Miss Pennyfeather," snapped Professor Binns. "I repeat, if the likes of Dumbledore -"
"But maybe you've got to be related to Slytherin, so Dumbledore couldn't -" began Dean Thomas, but Professor Binns had had enough.
This was evidently correct - there were apparently magical protections in place such that it couldn't be found by anyone but Slytherin's Heir.
Of course, Harry was able to find it and open it because
he was one of Voldemort's Horcruxes, and Voldemort was Slytherin's Heir.
Ginny was able to open it because she was being possessed by Voldemort's diary when she opened it.
Also, as others have indicated, speaking Parseltongue was a very rare inherited ability that was strongly associated with the Slytherin family. As far as I can recall, the only actual examples in the series of people who could speak Parceltongue were Harry and various descendants of Slytherin. Dumbledore was evidently able to understand it to some degree, but wasn't able to speak it; Ron Weasley managed to open the Chamber of Secrets at one point, but only with a weak imitation of Harry's Parceltongue, and that was only after he already learned where it was and how to open it with assistance from Harry and Hermione - he wouldn't have been able to discover where it was or how to open it without help from a Parcelmouth like Harry.
In the "modern" era of the school, there is another part to the answer. If anyone had seriously investigated the death of Moaning Myrtle, learned that she died when a boy entered the bathroom and summoned a monster, it probably would have been found. Hagrid was blamed, but could his spider have killed her the way a basilisk does? Wouldn't she have recognized the step and voice of a half giant? And how would the spider have gotten into the bathroom?
But no one seems to have asked her. The secret was preserved because it was easier to blame the half giant, and no one cared enough about an annoying girl to press for a better answer.
To drop another possibility, the Chamber itself was opened by Ginny via the diary. At that point it was then "open", and Harry - and later Ron - merely caused the entrance to be revealed.
Before that point, the entrance was (magically) sealed, presumably following the departure of Riddle from the school.
So even if anyone had looked for the entrance in the bathroom, it wouldn't have been apparent, and most likely even Dumbledore wouldn't have been able to bypass the protective charms laid down by Slytherin himself.
From my understanding of having read the books, only the descendant of Slytherin or a Parselmouth could open the Chamber of secrets. The Gaunts(the family of Voldemort/Tom Riddle's mother) were the last direct descendants of Slytherin, and Tom the only "heir". Tom Riddle was able to open the chambers, and use the basilisk to do his bidding. Another point to note is that from the time of Tom Riddle to that of Harry Potter, no possible heir of Slytherin came to Hogwarts, since the last heir of Slytherin was Tom, and he had no children. Parseltongue is also equally rare.
From the movie adaptation of(and definitely the books, but I am a bit rusty when it comes to that) Chamber of secrets, since the creation of the room was about 1000 years before the timeline of the series, most adults regarded it as a myth, and hence, didn't go about looking for it,similar to the hallows.
When it comes to Myrtle,she was known to be a gossip, and maybe, her story of what actually killed her was (unjustly and quite wrongfully) thought of as her way to create more rumors, even in death.
Maybe these all together were the reasons why no one looked for the chambers.
The entrance is one of the things that is problematic if everything goes according to canon.
If you assume -like some of fanon does- that there are evil underpinnings then:
- The headmasters knew where it was.
<insert excuse here>(ie. they had no desire to tackle a Basilisk. -but also- they could not fathom sealing the door to a restroom... yeah, hmm)
- The headmasters did not know where it was, and two students managed to discover it (Tom Riddle and Harry Potter), plus Ginny
with Diaryand Ron
with unexplained loose end ability. The former two are hero/anti-hero leading roles so we can assume exceptionalism for story plot purposes.
Both have loose ends, 1. is more amusing if you allow for an Evil Dumbledore. 2. works for canon.
Ron opened it because:
- He was a parseltongue all along. And Ron lied to Harry (and obliviated Hermione?) Ron's character becomes independently problematic inside canon in this case.
- Dumbledore changed it so it does not require parseltongue, then told Ron how. And Ron lied to Harry (and obliviated Hermione?)
- Hissing was all it took, the protections were weak and somehow nobody else did any hissing near there. Inside a girl's bathroom. In a school. Indeed.
Maybe I just like fanfiction that twists the plot to fill in the giant plot-holes a bit too much. The HP books are particularly good on having such loose ends, which fanfiction authors love to play with.
The possible canon answer
Other answers partially mentioned this: Tom himself*, Ginny had the Diary*, Harry had a scar* and Ron / Hermione were carrying the cup*. So we may assume that all is well with canon and some form of Soul magic was involved.
In this scenario you would be free to disregard the parseltongue problem. (But Word of God requires it, so..)
Only the heir can enter... or, sigh ...fine! yes, yes, a fragment of their soul will do.