Gemara

17b (Abaye): One may dig a pit at the edge of his property if his neighbor does not have a pit nearby;

(Rava): No, he must distance three Tefachim from the border.

Version #1: In a kind of field in which people generally dig pits, all agree that he must distance from the border. They argue when it is the type in which people generally do not dig pits.

Abaye allows him to dig at the edge, for in such a field, we do not expect that the neighbor will want to dig a pit (nearby). Rava says that he must distance three Tefachim from the border. Just like he decided to dig a pit in such a field, perhaps his neighbor will also want to.

Version #2: In a type of field in which people do not generally dig pits, all agree that he may dig up to the border. They argue when it is the type in which people generally dig pits.

Abaye allows him to dig at the edge. Even Chachamim, who forbid planting a tree within 25 Amos of a neighbor's pit, say so only about an existing pit. Here, there is no pit when he digs.

Rava says that he must distance three Tefachim from the border. R. Yosi permits one to dig in is his own property, just like his neighbor may do so, without distancing himself. He says so only about planting, because the roots that will cause the damage are not present when he plants. Here, every jolt of the shovel softens the neighboring land, so he must distance himself.

(Mishnah): One must distance Gefes (refuse of olives), manure, salt... three Tefachim from his neighbor's wall, and apply plaster.

Question (against Rava in Version #1): This implies that if his neighbor did not have a wall there, he would not have to distance himself!

Answer: No, even if there is no wall there, he must distance himself. The Mishnah teaches that these things damage walls.

(Mishnah): One may not open a bakery or dyer's store or cattle pen under another's storehouse.

Question: This implies that if there was not a storehouse there, he would be allowed!

Answer: We are more lenient for necessities of living.

Support (Seifa): If the cattle pen preceded the storehouse, he may keep it there.

Rishonim

Rif and Rosh (2:1): The Halachah follows Rava. Even though we hold like R. Yosi, who permits one to dig in his own property, R. Yosi admits regarding Giri Dilei ('arrows', i.e. damage that comes directly from the person's hand). This is a case of Giri Dilei.

Rambam (Hilchos Shechenim 9:10): If Ploni wants to dig a pit at the edge of his property near his neighbor's (Levi's) field, if it is not normal to have pits in such a field, Levi cannot protest. If later, Levi decides to dig a pit, he must distance it three Tefachim from Ploni's pit, so six Tefachim will separate between the interiors of the pits.

Magid Mishneh: The Gemara did not explicitly say that when Levi decides to dig, he must distance six (between the interiors). His Rebbi and some Meforshim explain like this. Others say that he need not distance more than three. He can say 'it is enough that you did not distance from my border. Will you force me to distance extra within my property?!'

Rambam (ibid): If it is normal to have pits in a field like Levi's, Ploni must distance his pit three Tefachim from the border. When Levi wants to dig a pit, also he will distance three Tefachim from the border.

Rosh: What was the question against Rava from distancing Gefes...? If there is no wall now, there is no damage now! Tosfos answered that these things damage the land, and even after they are removed, the land will not be good for a wall. Alternatively, it is a toil to remove them, and perhaps when Levi builds his wall, Ploni will be sloth to remove the damager. R. Yonah says that we are concerned lest Ploni make a Chazakah of having the damager there for three years, and claim that Levi permitted it. Rava reason is not because every jolt damages even now. We said so only to show that even R. Yosi could agree. We did not need it according to Chachamim. Therefore, Ploni may not open a window opposite Levi's ruin. Perhaps when Levi wants to rebuild, Ploni will say that he has a Chazakah to have a window there; also, Levi will need to distance four Amos! I disagree. If one may put Gefes near the border when there is no wall or pit, Ploni cannot claim Chazakah, for until now he did not damage, so Levi could not protest. When one cannot protest, there is no Chazakah. Therefore, a window opposite a ruin has a Chazakah only if it was there three years before the house was ruined. If one had a window opposite a house, and the house became a ruin, it is advisable to have witnesses write for him that he had the window three years before it became a ruin.

Rosh (3): Rava agrees with Abaye that one need not distance if now there is nothing that would be damaged. In this case, even Chachamim forbid only a pit, for each jolt weakens the land, even if Levi has no pit now. Rava said that even R. Yosi agrees, because every jolt damages even now. He did not mean that only R. Yosi says so. Also Chachamim forbid only a pit, for this reason.

Piplulei Charifta (4): This is the conclusion. Above, the Rosh said that in the Sugya above, this reason was needed only for R. Yosi.

Beis Yosef (CM 155 DH u'Mah she'Chosav Afilu): The Gemara explained why it was necessary to teach both an irrigation channel and a laundry ditch. If a pit must be distanced only if it has water, the Gemara should have explained why it was necessary to teach these and also a pit!

Hagahos Ashri: When two things damage each other, and Ploni put his close to the border, also Levi may put near him. Since Ploni did slightly improperly, even though Levi could not protest against him, Ploni cannot stop Levi from doing so if Levi is willing to bear the damage.

Poskim

Shulchan Aruch (CM 155:18): If Ploni wants to dig a pit at the edge of his property near his neighbor's (Levi's) field, if it is not normal to have pits in such a field, Levi cannot protest.

SMA (42): It is normal to have pits in a field that must be irrigated, but not in a field that prospers through rain alone.

Shulchan Aruch (ibid): If later, Levi decides to dig a pit, he must distance it six Tefachim from Ploni's pit, so six Tefachim will separate between the interiors of the pits.

Beis Yosef (DH u'Mah she'Chosav Rabeinu): This is like the Rosh and Rambam. Hagahos Ashri disagrees.

Gra (47): The Gemara asked from opening a store or cattle pen under another's storehouse. This shows that when one may put a damager nearby, it is like a store or pen, or an opening opposite an opening (154:3 and Gra 154:66. The latter must distance enough to avoid damage.)

Shulchan Aruch (ibid): If it is normal to have pits in a field like Levi's, Ploni must distance his pit three Tefachim from the border. When Levi wants to dig a pit, also he will distance three Tefachim from the border.

Beis Yosef (DH ha'Ba): We rule like Rava, according to Version #2.

Beis Yosef (DH Kosav ha'Rashba): A case occurred in which Reuven dug a storehouse near the wall of Shimon's house without distancing in certain places. Shimon demands that he fill in three Tefachim where he did not distance, and plaster where he distanced. The Rashba (3:157) said that the questioner assumes that the distance in the Mishnah is even without moisture. If so, surely Reuven must do like Shimon says. However, some explain that the distance in the Mishnah is due to moisture. If so, when there is no moisture, one must distance like experts say is enough to prevent damage.