Gemara

R. Yakov saw a case in which a man asked his son to bring to him eggs from the top of a building. The son (sent the mother and) took the eggs, and died on his way down.

Question: R. Elazar taught that Shomer Mitzvah Eino Nizuk (harm will not befall someone doing a Mitzvah), not even on the way back!

Answer: He went down a rickety ladder. The above law does not apply to dangerous situations;

"Shmuel said "Eich Elech v'Shama Sha'ul va'Haragani'" (he was concerned for danger, even though Hash-m told him to anoint David, which is a Mitzvah)!

Pesachim 31b (Mishnah): If a house collapsed on top of Chametz, it is as if Bi'ur (eradication) was done.

R. Shimon ben Gamliel says, this is when a dog could not find the Chametz.

(Rav Chisda): One must be Mevatel it.

(Beraisa): A dog can find (buried Chametz) up to three Tefachim (deep).

8a (Beraisa): One who checks for Chametz need not stick his hand into holes and cracks, due to danger.

Suggestion: It is dangerous due to scorpions.

Question: If so, one would not use it!

Defense: The case is, (Chametz was used in a building, and) it fell. (Scorpions are often found amidst the rubble.)

Question: If so, it need not be checked (at all)!

(Mishnah): If a house collapsed on top of Chametz, it is as if Bi'ur was done.

Answer: That is when a dog could not find the Chametz.

Question: (Why does danger exempt?) R. Elazar taught that Shomer Mitzvah Eino Nizuk!

Answer #1 (Rav Ashi): Perhaps he lost a needle, and he will look (also) for it (at the time of Bedikah).

Question (Beraisa): If one gives to Tzedakah 'in order that my son should live', or 'in order that I will merit the world to come', he is a perfect Tzadik. (Likewise, one who looks for Chametz and a needle is a Shali'ach Mitzvah!)

Answer: Perhaps he will look for a lost needle after the Bedikah.

Answer #2 (Rav Nachman bar Yitzchak - Beraisa): If there is a hole in a wall between Reuven and a Nochri, Reuven checks as far as his hand reaches. He is Mevatel what he cannot reach;

Flimo says, he does not check a hole at all, due to danger (lest the Nochri accuse him) of witchcraft.

Question: If so, Reuven would not use it!

Answer: Reuven uses it during the day, by sunlight without a Ner. The Nochri cannot accuse him of witchcraft. Bedikah is at night, by light of a Ner. The Nochri would accuse him of witchcraft.

We do not say that Shomer Mitzvah Eino Nizuk when danger is common.

Tosfos (8a DH Hacha): When a dog can unearth it, one is exempt only due to danger. Regarding a wall that fell, the Gemara connotes that if a dog can unearth it, it is not considered Mevu'ar, and one must be Mevatel it. He is not exempt due to danger of scorpions. The Ri says that when there is Vadai Chametz, Chachamim obligated him to hire workers with an ax and shovel. They did not impose this toil if it is a Safek whether there is Chametz there.

Rishonim

Rambam and Rosh (Hilchos Chametz 2:5 and Pesachim 1:13): If there is a hole in the wall between Reuven and Shimon, each checks as far as his hand reaches. He is Mevatel the rest. One does not check a hole at all if a Nochri is on the other side, lest the Nochri think that the Yisrael is doing witchcraft against him. Rather, he is Mevatel it in his heart, and this suffices.

Rosh: Even though Shomer Mitzvah Eino Nizuk, where there is danger is different. Since there is danger, we do like Flimo.

Poskim

Shulchan Aruch (OC 433:7): If there is a hole in the wall between Reuven and Shimon, each of them checks as far as his hand reaches, and this suffices. One need not check a hole if a Nochri is on the other side, lest the Nochri think that the Yisrael is doing witchcraft, and he will come to danger.

Bach (9): The printed versions of the Rambam say that one need not check at all, like Flimo. The original manuscript of the Rambam does not mention the law of a hole between a Yisrael and a Nochri. This is why the Magid Mishneh says that the Rambam holds that the law is the same as for a hole between Yisre'elim. However, Semag, Semak, the Ran, Roke'ach and R. Yerucham rule like Flimo. We hold like them and the Rosh and (printed versions of the) Rambam.

Taz (6): Seemingly, one need not check even if there is Vadai Chametz there, due to danger. Even though regarding a rockpile, if there is Vadai Chametz one must unearth it with an ax and shovel, there is different, for he can do it without danger. Here, there is no way to get the Chametz without danger. Really, also here one must check for Vadai Chametz, i.e. during the day. This is clear from the Gemara. It says that he uses it in daylight, so the Nochri does not suspect him of witchcraft.

Magen Avraham (12): The Bach asked why we do not say that we are concerned lest he look for a needle, like we say in the next Sa'if. He answered that here, one need not remove even Vadai Chametz. I disagree. One uses a hole every day. We are not concerned lest specifically now he will want to search for his needle. The next Sa'if discusses a wall that fell. He does not use it any more. There, perhaps now that he searches for Chametz, he will also look for his needle.

Gra (DH veshe'Bein): The Rosh says that when there is danger, we follow the lenient opinion.

Mishnah Berurah (30): On the day of the 14th, he checks in the daylight. The Nochri will not suspect him.

Kaf ha'Chayim (58): Some say that he must check during the day only if there is Vadai Chametz there. Some disagree. In any case one does not use a Ner even during the day.

Mishnah Berurah (31): Even though Chachamim exempted him from checking at night, he must do Bitul. The Acharonim say that even for Vadai Chametz, he does not check at night, only during the day, but he must do Bitul.

Kaf ha'Chayim (60): Shulchan Gavoha says that if the Nochri understands our Mitzvah and will not suspect him of witchcraft, he checks normally. The same applies if the hole is sealed on the Nochri's side, and he will not see the light from the Yisrael's Ner.

Shulchan Aruch (8): If one used Chametz in holes in a wall, and it fell and it is now a rockpile, even if it is not three Tefachim tall and a dog can find the Chametz, one need not check under it, since there is danger due to scorpions, which are found in rockpiles. We are concerned lest after finishing checking, when he is not engaged in a Mitzvah, he will seek a needle that he lost, and he will come to danger. This refers to Stam. If he knows that there is Chametz underneath, if there is not three Tefachim on top of it, he must unearth it with an ax and shovel in a way that there is no danger. If there is three Tefachim on top of if, he is Mevatel it in his heart, and this suffices.

Beis Yosef (DH Kosel): Rashi explains that if the building fell, one merely looks for visible Chametz. He need not stick his hands amidst the rocks, due to danger of scorpions. The Tur holds like Tosfos, that Chachamim exerted people to hire workers to unearth with an ax and shovel only when there is known Chametz. The Rif and Rosh only brought the Mishnah (about a house that caved in) and what the Gemara on 31b, but not the Sugya in 8a. This is astounding. Even though Rav Nachman bar Yitzchak established the Beraisa differently, that the danger is due to Nochrim, Answer #1 was not rejected. Rav Nachman bar Yitzchak could agree to the law! Rav Ashi answered the difficulty. This shows that he holds like it!

Bach: Why didn't the Beis Yosef ask why also the Rambam, Semag, Semak, Agudah, R. Yerucham and Kolbo omitted the Sugya on 8a? I say that they do not distinguish whether or not there is Vadai Chametz, unlike the Ri. In any case it is not considered Mevu'ar if a dog can unearth it. The Beraisa on 8a exempts from sticking his hands in, and we say that this is when a dog could unearth it. It does not exempt from Bi'ur. Rather, one need not do a proper Bedikah with a Ner. Rather, he hires workers to remove the Chametz with an ax and shovel. Why did the Tur rule like Tosfos against all those who disagree, and did not mention their opinion?!

Taz (8): Rashi says that one must be Mevatel the Chametz, lest it be unearthed during Pesach, and he will transgress. Semak says that he must be Mevatel, lest he transgress hiding away Chametz. It seems that this is even if it will not be unearthed. Rashi disagrees, for he did not actively hide it. Also the Shulchan Aruch obligates Bitul. Ri'az (Shiltei ha'Giborim 9b, 2) says that when there is three Tefachim on top of it, it is permitted after Pesach. I say that if there was a wall, and it fell, surely he will clear the rockpile to fix his wall. This is like hiding Chametz in a pit. If rocks fell in a way that it is not clear that they will be cleared away, it is permitted after Pesach if he did Bitul.

Magen Avraham (13): This is only where scorpions are common. This is why the Rif and Rosh omitted this. Perhaps where snakes are common one must check, for the danger is not so clear.

Mishnah Berurah (35): In a place where snakes kill, they are like scorpions.

Magen Avraham (15): We are concerned for a needle lost beforehand. While checking, Shomer Mitzvah Eino Nizuk, even monetarily (so he would not lose a needle).

Taz (7): The Maharshal says that Rashi explains that he lost the needle before the Bedikah. I.e. we cannot say that he lost it due to the Bedikah, for Shomer Mitzvah Eino Nizuk. Rather, he lost it beforehand, and perhaps he will look for it after finishing checking, while his Ner still burns. The Beis Yosef asked why Poskim did not distinguish between Vadai and Safek Chametz. He holds that one must distinguish, due to Tosfos' question (why this is unlike a house that caved in). Really, Rav Nachman bar Yitzchak disagrees with Answer #1. He holds that Shomer Mitzvah Eino Nizuk protects even if he looks for a needle afterwards. Therefore, he explains that the danger is due to Nochrim. He can say that even when there is danger of scorpions, one must check even for Safek Chametz. The Poskim who did not distinguish hold like him.

Mishnah Berurah (33): Even though a dog could smell it and unearth it and reveal it, one need not check for it.