[A cavity] one handbreadth by one handbreadth squared that is one handbreadth high conveys tumah and acts as a barrier against tumah. How so? [In the case of] a sewer beneath a house that contains a [cubic] handbreadth and has an outlet of a [square] handbreadth — [if there is] tumah in-side [the sewer], the house remains tahor; [if there is] tumah in the house, whatever is inside [the sewer] remains tahor, because it is the way of the tumah to exit, but it is not its way to enter. [In the case where] the sewer contains a [cubic] handbreadth and its outlet is smaller than a [square] handbreadth — [if there is] tumah inside [the sewer], the house becomes tamei; [if there is] tumah in the house, whatever is inside [the sewer] remains tahor, because it is the way of tumah to exit, but it is not the way of tumah to enter. [In the case where the sewer is] smaller than a [cubic] handbreadth and its outlet is smaller than a [square] handbreadth — [if there is] tumah inside [the sewer], the house becomes tamei; [if there is] tumah in the house, whatever is inside [the sewer] becomes tamei. [This law is true] whether the hole was hollowed out by water, by small ani-mals, or by erosion, and [is also true if it is formed by] a pile of boulders or a stack of beams. R’ Yehudah says: Any roof that is not man made is not considered an ohel. However, he agrees with respect to crags and projecting rock formations [that they are considered ohalim].