Gemara

63a (Mishnah): If a Nazir shaved (upon completing Nezirus) and then learned that he was Tamei due to a known Tum'ah, the days he counted are Batel. 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.

63b (Beraisa): If one went on a road and underneath was found a Mes spanning the width, he is Tamei for Terumah, but Tahor for Nezirus or an Oseh (one who offers) Pesach.

This is only if it spans the entire width. If not, he is Tahor even for Terumah.

This is 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, this leniency does not apply. The coffin joins up the pieces, so he is Tamei.

This is 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.

This applies only to Tum'as ha'Tehom. If the Tum'ah was known, he is Tamei in all three cases.

64b (Rav Hamnuna): If a Nazir or an Oseh Pesach passed over Tum'as ha'Tehom on his seventh day of Taharah from Tum'as Mes, he is Tahor. Tum'as ha'Tehom is not strong enough to uproot (a Nazir's count or Oseh Pesach).

Pesachim 81b - Version #1 (Mar bar Rav Ashi): Ritzuy (acceptance of Korbanos in a case) of Tum'as ha'Tehom is only if he found out after Zerikah.

Objection (Beraisa): One who finds a Mes across the width of a path (on which he walked) is Tamei for Terumah, but Tahor for Pesach and Nezirus.

Tamei and Tahor refer to the future (he may be Zorek if it was not done yet)!

Version #2: Rather, Mar bar Rav Ashi taught that Ritzuy of Tum'as ha'Tehom is even if he found out before Zerikah.

Rishonim

Rambam (Hilchos Korban Pesach 6:11): If one went on a road and underneath was found a Mes across the width, if it was Tum'as ha'Tehom, even though he is Tamei for Terumah, he is Tahor regarding Pesach. He slaughters and eats Korban Pesach, even though it is possible that he touched. Even if the Mes is complete and spans the entire width, he offers Pesach unless he definitely touched. This is only if he was walking, for it is possible that he avoided touching. If he was carrying a burden or riding, he is Tamei, even though it is Tum'as ha'Tehom, for he cannot avoid touching, moving or towering over the Tum'ah.

Kesef Mishneh: The Rambam explains that 'this is when he walked' refers to the Reisha, when the Mes spans the width of the road. Even so, it is possible that he veered off the path and did not touch, move or tower over the Tum'ah it. Rashi explains that it refers to the Seifa, when the Mes was in pieces.

Ri Korkus: Rashi holds that in the Reisha he is Vadai Tamei, nevertheless he is Tahor for Pesach, i.e. if he already offered it he was Yotzei. Rashi says that the text should not say 'if he was riding... he is Tamei even regarding Nazir and Pesach.' The Rambam's text said this, therefore he explains that Vadai Tum'as ha'Tehom is permitted only if one found out after offering Pesach. The Reisha discusses Safek Tum'as ha'Tehom, so he may offer Pesach. In Hilchos Nezirus, he discusses when it is possible to pass through b'Taharah with great difficulty. Alternatively, one who walks could go on a hill or canyon to the side of the road, but one who rides or carries a load cannot. Alternatively, even if he Vadai towered over the Tum'ah, it is a Safek, for perhaps it was a Nochri, who is not Metamei b'Ohel. This is why the Rambam did not say 'perhaps he did not tower over it.' He says 'if he was riding... he cannot avoid touching, moving or towering.' This refers to when we know that the Mes was a Yisrael. Even if he could have avoided touching or moving, if he surely towered he is Tamei.

Shev Shematsa (5:3 DH Ach): The Rambam holds that Tum'as ha'Tehom is Metaher only a Safek. This is even if a majority supports the Tum'ah, for he is Tamei for Terumah. All the more so, Tum'as ha'Tehom is Metaher against a Chazakah! This is unlike Perush ha'Mishnayos! Orach Mishor says that in Hilchos Avos ha'Tum'ah 18:1, the Rambam says that Tum'as ha'Tehom helps even for Vadai Tum'ah. He retracted what he wrote here.

Rambam (12): If one Muchzak to be Tahor offered Pesach, later found out that he was Tamei through Tum'as ha'Tehom, he is exempt. This is a tradition.

Ri Korkus: The Gemara concluded that there is Ritzuy even if he found out before Zerikah! Rashi said that if he found out beforehand, he is postponed to Pesach Sheni. He must say that if Zerikah was done, he was Yotzei. This is difficult. Also, the Gemara explained that the Beraisa (which says that he is Tahor) discusses before Zerikah! Rather, when Rashi says 'after offering Pesach', he means after slaughter and before Zerikah. We must say the same for the Rambam. 'If one offered...' means 'if one slaughtered.'

Rambam (Hilchos Nezirus 9:18): If a Mes was across the width of a road, even if the only way to pass is over it or to touch by the side, even if it is a known Tum'ah, if a Nazir passed he is Tahor, for it is Safek Tum'ah in Reshus ha'Rabim.

Rebuttal (Ra'avad): This is very far from the Gemara.

Kesef Mishneh: The Rambam explains that 'this is when he walked' refers to the Reisha, when the Mes spans the width of the road. However, the Beraisa said that regarding a known Tum'ah, even one who walked is Tamei! We must say that the Rambam had a different text. Perhaps the Ra'avad asks why this is called Safek if he cannot avoid the Tum'ah. We must say that it is possible to avoid it with difficulty.

Question: If we are Metaher due to Safek Tum'ah in Reshus ha'Rabim, this should apply even for Terumah!

Answer #1 (Ri Korkus ibid.): Perhaps it is Tamei regarding Terumah, for it is almost Vadai.

Support (Lechem Mishneh): The Rambam (Hilchos She'ar Avos ha'Tum'ah 18:3) says 'this is unlike other Sefekos. The Chazakah is that he touched.'

Question (Lechem Mishneh): The Gemara equated Nazir and Pesach, and regarding Pesach the Rambam is not Machshir Vadai Tum'ah!

Answer #1 (R. Meir Simchah 63a [3]): The Gemara said that regarding known Tum'ah, all three are Tamei. This refers to walking, carrying a load or riding. It cannot refer to Pesach, Nazir and Terumah, for the Gemara discussed Ritzuy, which applies only to Pesach.

Answer #2 (to both questions - Malbim, Bamidbar 6:9 (109)): Nazir and Pesach are the same regarding Tum'as ha'Tehom. The Rambam is Metaher a Nazir because the Sifri expounds "v'Chi Yamus Mes Alav" to exclude a Safek.

Keren Orah 63a (DH Ela d'Od): A Safek Tum'ah (in Reshus ha'Rabim) is only mid'Rabanan. Chachamim forbid to slaughter Pesach due to Tum'ah mid'Rabanan, but they do not uproot Nezirus, even when it is close to Vadai.

Chazon Ish (Nashim 139:16): The Rambam holds that the law of Safek Tum'as ha'Tehom applies only to Pesach, but not to a Nazir.

Tosfos (63b DH l'Terumah): He is Tahor for Nazir and Oseh Pesach and Tamei for Terumah when there is no way to pass over without becoming Tamei. If it is possible to pass over without becoming Tamei, he is Tahor even for Terumah, for this is Safek Tum'ah in Reshus ha'Rabim.