We have before us Mishnah 8 in the third chapter of Tractate Bava Basra, the concluding Mishnah of the chapter. It does not deal with the laws of chazakah nor with hezek re'iyah, but since we were dealing with hezek re'iyah - damage caused to the public - the Mishnah now discusses other ways in which a person might cause damage to the public.
"Ein osin chalal tachas reshus harabim":
The Mishnah begins: "Ein osin chalal tachas reshus harabim boros shichin ume'aros" - one may not make a hollow space beneath the public domain, such as pits, ditches, and caves. It is forbidden to dig in the ground beneath the public domain, whether a pit, a ditch, or a cave. Generally, these are open spaces, at least the pit and the ditch, but the main point of the law is that one may not use this area: one may not build underground pools or water storage facilities beneath the public domain, out of concern that the ground will collapse and passersby will be harmed.
Rabbi Eliezer disagrees and "matir kedei shetehei agalah mehaleches ute'unah avanim" - permits it provided it is strong enough that a wagon loaded with stones can travel over it. If the hollow space is built properly, in a way that it can support even the heaviest load that passes through the public domain, such as a wagon loaded with stones, there is no longer a concern that someone will be injured, and the matter is permitted.
It emerges that the root of the dispute is the question of concern for the future:
Tanna Kamma: Even if the work was done properly from the outset, it only lasts for a certain amount of time. Eventually, the hollow space might collapse and someone could be injured, even if fifty years pass - and therefore, it cannot be permitted.
Rabbi Eliezer: If the matter is done properly in the present, that is sufficient, and there is no need to be concerned about the distant future.
The halachah was ruled in accordance with the Tanna Kamma. However, we should add the words of the Rema, that everything follows the local custom. Today, it seems that people agree to the existence of underground infrastructure that benefits the public, such as sewage pipes and the like, and since everyone agrees and waives their rights - the matter is permitted.
"Ein motzi'in zizin ugezuztra'os":
"Ein motzi'in zizin ugezuztra'os lereshus harabim" - one may not extend projections or balconies into the public domain. One may not extend projections, balconies, and the like from the private domain into or over the public domain, out of concern that passersby in the public domain will be injured: they might receive a blow to the head, crash into them with their vehicle, and so on. A person is not allowed to take over the airspace of the public domain.
"Ela im ratzah kones lesoch shelo umotzi" - rather, if he wishes, he may draw back into his own property and extend it. Someone who wishes to build a new balcony or stick out a flagpole must move his building line inward into his own property, so that even when the structure projects toward the public domain, it will not actually enter its airspace.
It is worth noting that this law applies only in a place where people might be injured. The Gemara explains that the intention is up to the height of a person riding a camel and sitting on its back, as traffic reaches up to there. Therefore, a third-floor balcony protruding over the public domain is permitted, since it does not disturb anyone - provided that two conditions are met:
That it does not harm passersby: that they will not bump into it and it will not disturb them on their way.
That it does not cast a shadow: one may not cause sunlight to be blocked from the public, and if the structure is located in a place that reduces the sunlight - this too is forbidden.
"Lakach chatzer u'vah zizin u'gezuztraos" - If one bought a courtyard containing projections and balconies:
The Mishnah, and with it the entire chapter, concludes with the law of one who buys a property that already has projections or balconies hanging over the public domain. May the public demand that he remove them from the public domain? To this, the Mishnah states: "Harei zo bechezkasah" - it retains its presumptive status. The court advocates on his behalf, saying: Presumably, this was the case from the outset, and the previous owner acted lawfully - whether he received permission or whether he recessed his building into his own property, meaning his boundary line was drawn back from the public domain and the projections stand entirely within his own territory. Therefore, the new owner is not obligated to remove his balcony or similar structures.
However, this allowance stems from the fact that the true reality is unknown to us. Therefore, if the owner wishes to expand his house and occupy the area beneath the balcony, he is not permitted to do so, lest this area belongs to the public domain. We find that due to the doubt, he is not obligated to retreat or remove his projections, but neither may he rely on that assumption to expand his construction and occupy the ground beneath the projections.
In summary: This Mishnah deals with damage to the public that is not in the form of visual damage - beneath the ground and above it. We learned the prohibition against creating a hollow space beneath the public domain and the dispute between the Tanna Kamma and Rabbi Eliezer regarding the concern for the future, in which the halachah follows the Tanna Kamma, and the words of the Rema that everything follows the local custom. Likewise, we learned the prohibition against extending projections and balconies into the airspace of the public domain, the solution of recessing into one's own property, the height limit of a camel rider, and the two conditions of preventing damage and preventing a shadow. Finally, we discussed the law of one who buys a courtyard containing projections and balconies, that it retains its presumptive status out of doubt - and we do not require him to remove them, but neither do we allow him to expand based on this.