(Mishnah): If a Nazir shaved (upon completing Nezirus) and then learned that he was Tamei:
If it was a known Tum'ah, the days he counted are Batel (and he must bring Korbanos of a Nazir Tamei, and be a Nazir again);
If it was Tum'as ha'Tehom (an unknown Tum'ah), he has fulfilled Nezirus.
If he found out before shaving, in either case the days he counted are Batel.
For example, if he went to immerse in a Mikvah in a cave, and later, Tum'ah was found floating on the water, he is (assumed to be) Tamei;
If Tum'ah was found buried in the floor of the cave, we distinguish:
If he was Tahor, and entered the water merely to cool off, he is still Tahor;
If he was Tamei Mes, and immersed to complete his Taharah, he is Tamei.
There is good reason to leave him in his Chazakah (status quo of Tahor or Tamei).
(Gemara) Question: What is the source that Tum'as ha'Tehom does not nullify Nezirus?
Answer #1 (R. Elazar): "If a Mes will die upon him (a Nazir)" - only if it is clear to him.
Answer #2 (Reish Lakish): "If a man will be Tamei Mes, or on a far away Derech (road, and he cannot offer the Korban Pesach)" - just like a road is exposed, only exposed Tum'ah disqualifies an Oseh Pesach (Rambam - one about to offer the Korban Pesach; Rashi - one who offered (Tosfos - slaughtered) it.) (We learn Nazir from Pesach.)
Question #1 (Mishnah): Tum'as ha'Tehom is one that no one knew about. If one person anywhere in the world knew, it is not Tum'as ha'Tehom.
This is like Reish Lakish.
According to R. Elazar, even if someone knew about the Tum'ah, if it was not clear to the Nazir, it should not cancel Nezirus!
Question #2 (Beraisa): If one found a Mes lying across the (entire) width of the path (on which he had walked), he is Tamei regarding Terumah and Kodshim, but Tahor regarding Oseh Pesach and Nezirus.
Why should there be any difference? (Each opinion had one source, Nazir or Pesach. If we learn from it to the other, we should also learn to Terumah and Kodshim!)
Answer #3: Rather, Tum'as ha'Tehom is a tradition from Sinai.
(Mishnah): If he found out before shaving (in either case the days he counted are Batel).
Question: Who is the Tana of the Mishnah?
Answer (R. Yochanan): It is R. Eliezer, who says that the Isurim of Nezirus are in effect until he shaves.
FROM WHEN IS HE TAMEI?
Question (Rami bar Chama): If he became Tamei before completing Nezirus, and learned of this after completing Nezirus, what is the law?
If it depends on when he found out, this was after completion, so he fulfilled Nezirus;
If it depends on when he became Tamei, this was before completion, so he must be a Nazir again.
Answer (Rava - Mishnah): If he learned before shaving, in either case days are Batel.
Question: When did he find out?
If this was before completing Nezirus, there is no need to teach this!
Answer: Rather, it was after completing Nezirus (and days are Batel, i.e. we go after when he became Tamei).
Question: We can still ask, are all the days Batel, or only seven?
Answer: According to Chachamim, all the days are Batel. According to R. Eliezer, Tum'ah after completing Nezirus nullifies only seven days.
Question: Perhaps R. Eliezer said so only when he became Tamei after completion. Here, he became Tamei during Nezirus!
Or, perhaps it depends on when he found out, and this was after completion.
Answer (Mishnah): In either case, he nullifies.
The Mishnah does not distinguish if the Tum'ah was before or after completion (and in the latter case, only seven days are Batel).
WHAT IS TUM'AS HA'TEHOM?
(Beraisa): If a Mes was found underneath a road, spanning the width of the road, one who traversed this road is Tamei regarding Terumah, but Tahor regarding Oseh Pesach or Nezirus.
This applies only if it spans the entire width. If not, he is Tahor even regarding Terumah. (A Safek Tum'ah in Reshus ha'Rabim is Tahor.)
Also, law (a) applies only if the Mes is whole. If the body is broken or in pieces, he is Tahor even for Terumah, for perhaps he walked between the pieces.
If the body is in a coffin, leniency (c) does not apply. The coffin joins up the pieces, so he is Tamei.
Also, leniency (c) applies only if he was walking. If he was carrying a burden or riding, he is Tamei.
One who is walking can avoid touching, moving or towering over the Tum'ah. One carrying a load or riding cannot.
Law (a) applies only to Tum'as ha'Tehom. If the Tum'ah was known, he is Tamei even regarding Pesach and Nezirus.
Tum'as ha'Tehom is a Tum'ah that no one in the world knew about it. If anyone knew about it, it is not Tum'as ha'Tehom.
If the Tum'ah was concealed in straw or stones, it is Tum'as ha'Tehom. If it was not seen because it was in water, it was dark, or it was in a crevice in a rock, it is not Tum'as ha'Tehom.
Tum'as ha'Tehom applies only to Mes (Rashi - Tum'as Mes; Rambam - to one who died, not to one who was murdered).
(Mishnah): If he descended to immerse...
(Doubtful) floating Tum'ah of a Sheretz is Tahor.
(Beraisa): A Safek (based on) Tum'ah floating in water is (assumed to be) Tahor, whether the water is in a Kli or in the ground;
R. Shimon says, if it is in a Kli, it is Tamei. In the ground, it is Tahor.