Gemara

(Beraisa): We mark off places of Safek Tum'as Ohel, such as Beis ha'Peras.

(Mishnah): If a grave was plowed, it makes a Beis ha'Pras.

Question: How much becomes a Beis ha'Peras?

Answer: It is up to the length of a furrow, 100 Amos.

Question: A Beis ha'Peras does not have Tum'as Ohel!

(Rav Yehudah): A person may walk through a Beis ha'Pras by bending down and blowing the dirt before each step (to clear away any small bones that might be there).

(Rav Yehudah bar Ami): If many people walked through a Beis ha'Pras, it is Tahor (we assume that any bones have been crushed to less than the Shi'ur to Metamei, i.e. the size of a barley seed).

Answer (Rav Papa): A Beis ha'Peras in which a grave was plowed does not have Tum'as Ohel, but a field in which a grave was lost has Tum'as Ohel.

Question: Do we call a field in which a grave was lost a Beis ha'Pras?

Answer: Yes!

(Mishnah): There are three kinds of Beis ha'Peras: a field in which a grave was lost, a field in which a grave was plowed, and a field of crying.

Question: What is a field of crying?

Answer (R. Yehoshua bar Aba): This is the place where, when a Mes is brought elsewhere for burial, those who brought it hand it over to those who will bury it.

Question: Why are we concerned for Tum'ah there?

Answer (Avimi): This is because they despair (if a limb fell off, they do not bother to look for it).

Temurah 12b (Mishnah): A Beis ha'Pras does not make a Beis ha'Pras.

This is unlike R. Eliezer:

(Mishnah - R. Eliezer): A Beis ha'Pras makes a Beis ha'Pras.

Text of Shitah - Chachamim say that Beis ha'Pras does not make Beis ha'Pras.)

Question: How far does Beis ha'Pras extend?

Answer (Rav Dimi): Three fields become Beis ha'Pras (until) the (standard) length of a furrow, 100 Amos;

(Beraisa): If a grave was plowed, it makes a Beis ha'Pras up to the length of a furrow, 100 Amos.

Rishonim

Rif and Rosh (Hilchos Tum'ah (after Menachos) 2b and 9 DH v'Heicha): A Kohen may not enter a Beis ha'Peras, which is a field in which a grave was plowed. A Beis ha'Peras extends 100 by 100 Amos, four times the area of a Beis Se'ah

Rambam (Hilchos Tum'as Mes 8:1): If a grave was lost in a field, its dirt has Tum'as Maga and Masa (one who touches or moves it becomes Tamei), like a Beis ha'Peras. Perhaps the grave was trampled and there are bones the size of a barley seed in the dirt. One who towers above any part of the field becomes Tamei. If one erected an Ohel in the field everything inside is Tamei, lest it towers over the Tum'ah.

Rambam (9:12): All of these Tum'os due to Safek are mid'Rabanan.

Note: In Perek 9 the Rambam discussed one who finds three graves and must be concerned for a cemetery, a Tamei mound that became mixed with Tahor mounds and a pit in which a Safek Nefel was cast. The previous Perek discussed a grave lost in a field, and the coming Halachos discuss a field in which a grave was plowed.

Rambam (10:1): A Beis ha'Peras is a field in which a grave was plowed. The bones were ground up and spread throughout the field; Chachamim decreed Tum'ah on the entire field. One hundred Amos by 100 Amos from the place of the grave becomes a Beis ha'Peras.

Rambam (2): The entire square, four times the area of a Beis Se'ah, becomes a Beis ha'Peras. The dirt has Tum'as Maga and Masa, but not Tum'as Ohel. One who towers over it is Tahor.

Rambam (3): If one plowed more than 100 Amos, past 100 Amos is Tahor, because the bones do not reach there.

Rosh (Ohalos 17:1 DH Malei): We cannot say that the entire field of 100 Amos by 100 Amos is Tamei. We are not concerned lest the plow take a bone more than 100 Amos in any direction, all the more so we are not concerned lest it take it more than 100 Amos combined in two directions (e.g. 80 Amos south and 30 Amos east)!

Note: It seems that we are concerned if the total distance is at most 100, e.g. 70 Amos south and 30 east (if the field was plowed in these two directions). We are concerned for a triangle that is half the square.

R. Shimshon (Ohalos 17:3 DH O she'Nimtza): It is a fine to consider a field in which a grave was plowed to be Tamei. This applies to a Yisrael who plowed his own field, but not if one plowed another's field or if a Nochri plowed his own field.

Rashi (Temurah Sof 12b and 13a DH Shalosh): R. Eliezer holds that a Beis ha'Pras makes a Beis ha'Pras in all four directions. According to Chachamim, three fields become Beis ha'Pras: the field in which the Mes was lost, and the adjacent fields in the two directions in which the field is normally plowed, north-south or east-west.

Tosfos (Nidah 57a DH Melei): The Tosefta (Ohalos 17:1) says that 100 Amos in every direction becomes a Beis ha'Peras.

Aruch l'Ner (DH Melei): According to this, the Beis ha'Peras is 200 by 200 Amos. The Rambam does not bring the Tosefta. He explains that it argues with the Mishnah. It allows 100 Amos in all four directions. The Rambam rules like the Mishnah, which allows 100 Amos between width and length together. Therefore, if one plowed 100 Amos in one direction we are not concerned for the other directions.

Note: We are concerned for all four directions when we do not know in which direction the grave was plowed.

Tosfos (Temurah 13a DH Shalosh): R. Eliezer holds that wherever was plowed become a Beis ha'Peras, without limit.

Shitah Mekubetzes (Temurah 12b (15)): In the Beraisa, Chachamim say that Beis ha'Pras does not make Beis ha'Pras. The Gemara asks according to R. Eliezer how far a Beis ha'Pras extends.

Poskim

Shulchan Aruch (YD 372:1): A Kohen may not enter a Beis ha'Peras (a field in which a grave was lost and plowed; it extends 100 Amos by 100 Amos).