Nazir Chapter 2, Mishnah 2. The Mishnah opens with the words "Nazir sheGilach" - a nazir who completed his term of nezirus, brought his offerings, shaved his head, and did everything required of him, and afterward discovered that at some point during his nezirus he had been tamei. His ruling depends on the type of tumah:
Known tumah: a tumah that people are aware exists - even though he only found out about it after completing his nezirus, it invalidates his nezirus. He must begin the count again, bring the offerings of tumah, and count his nezirus over.
Tumas hatehom (tumah of the deep): a tumah that nobody knows about - buried deep, with no one aware that it is there. If he learned of it only after finishing his nezirus, it does not interrupt the nezirus and has no effect on him.
This ruling is a halachah received through tradition, and it does not necessarily have to fit logically within the framework of the laws of nezirus. It is a fact known to us: tumas hatehom is an exception to the general rules of tumah, and anyone who completed his nezirus and only afterward learned of it is unaffected by it.
One who has not yet completed his nezirus:
The Mishnah continues: if he learned that he became tamei before he completed his nezirus and brought his offerings - whether it was tumas hatehom or a known tumah - it invalidates and interrupts the nezirus.
A precise reading of the Mishnah's wording - the view of Rabbi Eliezer:
The Gemara notes precisely that the Mishnah used the wording that he had not yet shaved his head, and from here we learn that even if he already brought his offerings but did not shave, the tumah invalidates. This terminology follows the view of Rabbi Eliezer, who holds that the actual shaving is what determines whether a person has completed his nezirus. But according to the Chachamim, as long as he brought one of the offerings and its blood was thrown, tumas hatehom does not invalidate, since it is considered as though it occurred after the nezirus. The Mishnah, which makes the matter depend on his still needing to shave, follows the view of Rabbi Eliezer, that until everything required is done, including the shaving, the nezirus is not complete.
What is a case of tumas hatehom?
The Mishnah clarifies what a scenario of tumas hatehom looks like: "Yarad litbol bame'arah" - these caves contained mikvahs, similar to the mikvah of the Arizal in Tzefas today, and a person went down to immerse in them. "Venimtza mes tzaf al pi hame'arah" - after he immersed, a corpse was found floating on the surface of the water in the cave, and he is tamei. The reason: this is not tumas hatehom but rather a tumah that people are aware of, and it is not a hidden tumah.
But if "nimtza meshuka bekarka hame'arah" - the corpse was found sunk in the floor of the cave - this is a tumah that no one is aware of, and then the matter depends on the purpose of his immersion:
"Yarad lehakeir": if he entered the mikvah only in order to cool off, he is tahor, because this is tumas hatehom, and all this, as stated, is where he found out after the shaving.
He went down to purify himself from tumas meis: if he entered the mikvah in order to purify himself from tumas meis that was upon him, he is tamei, even if he did not find out about it until after he completed his nezirus.
In other words: a person who was tamei and went down to purify himself, and afterward counted his nezirus and completed it, and about a month later learned that already at that time there had been a corpse sunk beneath the mikvah - since the purpose of his going down into the mikvah was to purify himself from tumas meis, we do not say that he is tahor, but rather that he is tamei.
The reason - "shechezkas tamei, tamei, vechezkas tahor, tahor":
One who has a status of tumah - we say of him that he remains tamei; and one who has a presumed status of taharah is tahor. It emerges that the law of tumas hatehom, that if it was discovered after the shaving and after the nezirus ended he is tahor, was stated specifically regarding someone who did not have a presumed status of tumah at that time. But one who did have a presumed status of tumah at that time, when he encountered the tumah - even though it is tumas hatehom - remains in his tumah.
And this is the reason for the matter: "sheyeish leragel hadavar" - there is substance to the matter, there are grounds for it. There is a logical connection that one who was tamei remains tamei; and only regarding one who was tahor and encountered tumas hatehom did the halachah provide the basis that he not be considered tamei if he learned of it after completing his nezirus.
In summary: in this Mishnah we learned the law of tumas hatehom - a hidden tumah that no one knows about: if it was discovered after the nazir completed his nezirus and shaved, it does not invalidate his nezirus, whereas a known tumah does invalidate. If he learned of it before he completed his nezirus, even tumas hatehom invalidates. We also learned that the Mishnah's wording, which makes the matter depend on the shaving, follows the view of Rabbi Eliezer, while the Chachamim make it depend on the throwing of the blood of one of the offerings. And finally, that this law was stated specifically regarding one who had a presumed status of taharah, since "chezkas tamei, tamei" - and one who went down to immerse in order to purify himself from tumas meis remains in his tumah.