Gemara

Nazir 54a (Mishnah): If a Nazir became Tamei through any of the following, he does not shave, bring Korbanos Nazir Tamei, or lose the days he counted. He needs Haza'ah (of Mei Chatas) on days three and seven, and immediately resumes counting: Chutz La'aretz, a Revi'is of blood, Kelim that touched a Mes...

Question: Did Chachamim decree Tum'ah on (one who enters) the air of Chutz La'aretz? Or, did they decree on (one who towers above) the soil?

Suggestion: Tana'im argue about the decree on Chutz La'aretz. (R. Yosi holds that they decreed on the soil, and Rebbi holds that they decreed on the air):

(Beraisa - Rebbi): One who enters Chutz La'aretz in a box becomes Tamei;

R. Yosi b'Rebbi Yehudah says, he remains Tahor.

Rejection: No, all agree that they decreed on the soil;

R. Yosi holds that an Ohel that is Zaruk (moving) is considered an Ohel (the bottom of the box shields from Tum'as Ohel of the soil); Rebbi disagrees.

Version #1 (in Tosfos) Objection (Beraisa - R. Yosi): If a box full of Kelim was thrown over a Mes, the Kelim become Tamei. If the box was stationary, the Kelim remain Tahor. (This shows that he agrees that an Ohel Zaruk is not an Ohel!)

Version #2 - Support (Beraisa - R. Yosi): If a box full of Kelim was thrown over a Mes, the Kelim become Tamei. If the box was (carried by people or animals) on the ground, the Kelim remain Tahor. (This shows that he holds that an Ohel Zaruk is an Ohel.)

Rejection #2: Both hold that the decree was on the air. R. Yosi holds that Chachamim did not decree about entering Chutz La'aretz in a box, since this is uncommon. Rebbi holds that they decreed even in such a case.

Rejection #3: They argue about whether or not we decree lest he stick his head or the majority of his body outside the box.

Gitin 8a (Beraisa): The soil of Surya is Tamei, like that of Chutz La'aretz. One may enter Surya (in a box) and remain Tahor, like Eretz Yisrael.

(Beraisa - Rebbi): One who enters Chutz La'aretz in a box becomes Tamei;

R. Yosi b'Rebbi Yehudah says, he remains Tahor.

Even Rebbi is Metamei only in Chutz La'aretz proper. Chachamim decreed Tum'ah on the soil and the air. In Surya, they decreed only on the soil.

Chagigah 24b (Mishnah): In Yehudah, an Am ha'Aretz is believed about (Taharah of his) wine and oil the entire year...

25a - Inference: They are not believed in Galil.

(Reish Lakish): This is because there was a strip of Eretz ha'Amim between Galil and the Mikdash. (Taharos brought to the Mikdash would become Tamei).

Question: One could bring them in a box and avoid the Tum'ah!

Answer: Our Mishnah is like Rebbi (who holds that a box does not shield from Tum'as Chutz La'aretz).

Rishonim

Rambam (Hilchos Tum'as Mes 11:5): One who enters Chutz La'aretz in an airborne box becomes Tamei, because an Ohel Zaruk is not considered an Ohel.

Question (Lechem Mishneh Hilchos Nezirus 7:6): Since they decreed on the air, in any case he is Tamei! And why does the Rambam (11:6) say that one can enter Surya in Taharah in a moving box? Since it is not an Ohel, it is like walking on the ground!

Answer #1 (Lechem Mishneh): The Rambam's text of the Gemara concludes that all agree that they decreed about the air. The case is, the box is sealed. An Ohel Zaruk shields from Tum'as Avir only if it is considered an Ohel.

Rebuttal (Mishneh l'Melech): One answer in the Gemara says that they argue about whether or not we decree lest he stick his head out. This shows that the box is not sealed! However, in Perush ha'Mishnayos and Tum'as Mes 11:5), the Rambam says that there is Tum'as Avir without Gush, which supports the Lechem Mishneh.

Answer #2 (Mishneh l'Melech): In Shabbos it says that there is Tum'as Avir, i.e. when one towers over the ground. We never resolved (in Nazir) the law when one does not tower over the ground. The Rambam is lenient, for it is a Safek mid'Rabanan. In Surya there is no Tum'as Avir at all.

Answer #3 (Likutei Halachos, Nazir 21a Zevach Todah): The Rambam holds that Tum'as Avir does not obligate Haza'ah. Ohel Zaruk is not an Ohel, so entering in a box is like walking on the ground, so he needs Haza'ah.

Mishneh l'Melech (1, DH uv'Ohel): Even though the final answer in Nazir says that all agree that Ohel Zaruk is considered an Ohel, the Rambam rules that it is not, like the Sugyos in Chagigah and Eruvin.

Rambam (Hilchos Nezirus 5:18): If a Nazir entered Ohel ha'Mes in a box, and someone else removed the lid...

Question: This implies that he is Tahor before the lid is removed. In Hilchos Tum'as Mes, the Rambam rules that Ohel Zaruk is not considered an Ohel!

Answer #1 (Gilyon Maharsha YD 372 on Shach 2): The Rambam holds that Ohel Zaruk is considered an Ohel mid'Oraisa (therefore a Nazir is exempt if the lid is left on). Mid'Rabanan it is not an Ohel.

Question (Tosfos Eruvin 30b DH u'Mar): The children who went to fill water for Mei Chatas used to ride on doors on bulls to shield from Tum'as Ohel (Sukah 21a). According to Rebbi, an Ohel Zaruk is not an Ohel, so this does not help!

Answer #1 (R. Tam): Rebbi holds like R. Yehudah (Sukah 21a), who says that they used to ride on (fat) bulls without doors. A verse teaches that they are an Ohel (in any case) - "Uva'Atzamos v'Gidim Tesochecheni."

Answer #2 (Rashba): All agree that an Ohel on top of rocks or something living shields from Tum'ah, even if it is moving.

Answer #3 (Mishneh l'Melech 1 DH v'Da sheha'Tosfos): Eretz Yisrael is Muchzak to be Tahor. The Rambam holds that the door is an Ohel Zaruk, which does not help. Nevertheless, we require a door to show the stringency of Parah Adumah.

Questions (Tosfos Chagigah 25a DH she'Retzu'ah): If a strip of Chutz la'Aretz interrupts, how could Efer Chatas be brought to Galil? How could people of Galil come for the Regel without becoming Tamei for seven days?

Answer #1 (Tosfos): There was a thin (Tahor) road from Yehudah to Galil. It sufficed to transport Efer Chatas, but it was too narrow to bring wine and oil.

Answer #2a (Mishneh l'Melech ibid., DH Od Kosvu): The Rambam holds that one who goes through in a box does not become Tamei for seven days.

Answer #2b (Rav Ransborg 1): People can ride on doors on bulls, according to the Rashba (above). The Maharsha (Eruvin Reish 31a) says that Chachamim did not rely on this to transport Kodshim b'Taharah, for it is not common. They were not stringent about the Mitzvah of Aliyah l'Regel.

Mishneh l'Melech: We do not rely on doors to transport Kodshim b'Taharah, lest one stick the Kodshim out (past the door). This is not a concern in a box, so R. Yosi b'Rebbi Yehudah, who says that Ohel Zaruk is an Ohel, allows it.

Poskim

Shulchan Aruch (OC 409:1): An Eruv Techumim in a Beis ha'Peras is valid even for a Kohen, for he could enter in an airborne box, or blow (the dirt) and walk.

Bi'ur Halachah (DH Eino): Ri'az allows even in a cemetery, because an Ohel Zaruk shields from Tum'ah. The Halachah does not follow him.

Gra (DH b'Migdal): The Halachah follows Rebbi (Ohel Zaruk is not an Ohel) in Surya, which has Tum'as Ohel. A Beis ha'Peras has no Tum'as Ohel.

Mishnah Berurah (6): The box is carried by animals, to avoid moving bones.

Magen Avraham (5): The box must be at least three cubic Amos (so it is not Mekabel Tum'ah).

R. Akiva Eiger (on Magen Avraham 1): An Eruv in a cemetery in a building is invalid for a Kohen. He could not get there, for Ohel Zaruk is not an Ohel. In a lone grave it is valid even for a Kohen. Even though four Amos around is Tamei, this is mid'Rabanan, and a (Magen Avraham 5 - big) box suffices for this. (Any box suffices for a Beis ha'Peras, since it has no Tum'as Ohel.)

Bi'ur Halachah (DH Mipnei): Perhaps a big box is not needed even for a lone grave. The box is a mere stringency; nowadays we are all Temei'im, anyway.