Gemara

(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 the Korban Pesach or Nezirus.

This 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 a Mes.

Pesachim 80b - Question: What does this exclude?

Answer #1: It excludes Tum'as Sheretz of the owner, or of the Kohen.

Answer #2 (Rav Yosef): The Beraisa discusses Tum'ah of the owner regarding Pesach. It exclude Tum'as ha'Tehom of Zivah (retroactive Tum'ah due to a later sighting of blood or Zov. At the time, no one knew about it). It is not Meratzeh.

65a (Rav Yehudah): The Mishnah says 'Mes' to exclude someone murdered. (It does not join to establish a burial site.)

Rishonim

Rambam (Hilchos Nezirus 6:18): Tum'as ha'Tehom is a Tum'ah that no one in the entire world knows about it. It applies only to one who died, but not to one who was killed, for the murderer knew.

Rebuttal (Ra'avad): No, 'it applies only to a Mes' excludes Zivah.

Defense #1 (Radvaz): The Rambam agrees that 'Tum'as ha'Tehom applies only to a Mes' excludes Zivah, He excludes a murdered Mes from the Reisha, 'it is a Tum'ah that no one in the world knew about it.'

Rebuttal (Shirei Korban Nazir 44b DH Ha): Orach Mishor says that the Rambam holds that 'only Mes' excludes Tum'as Sheretz, not Zivah. He himself explains that sometimes Tum'as ha'Tehom applies to one murdered, e.g. by an arrow. We follow the majority (if probably no one knew, it is Tum'as ha'Tehom). This is astounding! Perush ha'Mishnayos (Nazir 63a) explicitly says that 'only for a Mes' excludes someone murdered. Also, the Gemara should have explained the distinction! Also, we should not be stringent due to a majority regarding Safek Tum'ah in Reshus ha'Rabim!

Defense #2 (Lechem Mishneh 6:18 and 7:3): In Nazir (15b), we permit Tum'as ha'Tehom of Zivah! The Ra'avad must say that the Tana in Pesachim disagrees.

Rejection (Shirei Korban, ibid.): In Pesachim we conclude that Tum'as ha'Tehom of Zivah was not permitted! Nazir 15b is a mere Dichuy.

Defense #3 (Lechem Mishneh 7:3): We learned (Nidah 27b) that Rekev applies only to a Mes, but not to a murdered Mes (for some blood is lacking)!

Radvaz: We do not exclude a murdered Mes unless it is clear that a person, and not an animal, killed him. Perhaps the murderer left before his victim died! (There is no Tum'ah until he dies!) Indeed, in such a case it is Tum'as ha'Tehom. We exclude when he was stabbed in the heart or his head was cut off, and the murderer knows that he died immediately.

Rambam (19): If an exposed Mes was found, this is not Tum'as ha'Tehom. If it was submerged in the ground of a cave with water over it, this is Tum'as ha'Tehom, for it was unknown. If it was buried in straw or pebbles, this is Tum'as ha'Tehom. If it was in water in the dark, or in crevices in rocks, it is not Tum'as ha'Tehom.

Kesef Mishneh: Rashi explains that straw and pebbles often move from place to place, e.g. through wind, so it is possible that (the Mes was not buried by man, and) no one knew about it. Since people can see through water or holes in rocks, perhaps someone saw the Mes, so it is not Tum'as ha'Tehom.

Perush ha'Mishnayos (Nazir 63a): Tum'ah buried in the ground is Tum'as ha'Tehom, for we can say that no one knew about it. We cannot say so about floating Tum'ah, for it is readily available to be seen. When it is impossible for people to see it, we consider it to be Tum'as ha'Tehom until we find that someone knew about it. If we find exposed Tum'ah, we do not say that person no one saw it.

Orach Mishor (Nazir 63b DH b'Teven): The text of Rashi should say 'since people can see... it is impossible that no one saw the Mes, so it is not Tum'as ha'Tehom.' It is possible that someone saw Tum'as ha'Tehom, so we must say that a Mes is not Tum'as ha'Tehom when surely it was seen. The correct text of Perush ha'Mishnayos should say 'when it is possible for people to see it, it is Tum'as ha'Tehom...' Why did the Tosfos Yom Tov cite Perush ha'Mishnayos to say 'if it is impossible that no one saw it, it is Tum'as ha'Tehom'? Clearly, this text is mistaken!

Me'iri (Pesachim 80b DH ha'Pesach): Tum'as ha'Tehom is a Tum'ah that no one knew about, as if it were in the Tehom (depths). E.g., a wall fell on someone at night, and this was discovered only after he brought his Konamos.

Toras ha'Kodoshim (in Likutei Halachos) Pesachim 14b (100): Tum'as ha'Tehom is only if covered and impossible to see, like the Tehom.