One of the cases that the Mishna (15a) rules is not a good sukka is when someone hollows out a stack of grain to create a space for sleeping or eating. Even though the roof is made out of materials that ordinarily can be used for skhakh – roofing – a sukka cannot be made this way.
The Gemara on our daf quotes a baraita that seems, however, to teach the opposite. According to the baraita, someone who burrows into a stack of grain and creates an area large enough for a sukka can succeed in establishing a kosher sukka.
To solve this apparent contradiction, Rav Huna distinguishes between a situation where there was an already existing space within the mound that was a tefa high and seven tefahim in width and length, and one where no such space existed. In the event that there was an already existing space, it can be enlarged to create a sukka. The teaching in our Mishna was that in a case where the mound was solid, a person cannot dig out the space for a sukka.
The logic behind Rav Huna’s distinction is that, in the case where there was already an existing space of appropriate size, even if it could not be a sukka, it still had the halakhic designation of an ohel – an enclosed area. All that needs to be done is to widen the space – that is already acknowledged as being significant in the eyes of the halakha – so that it will be appropriate for use as a sukka. If there is no existing space, however, the sukka is not seen as having been made properly since the skhakh is in place even before the ohel inside exists – referred to as ta’eseh ve-lo min ha-asuy, meaning that the skhakh must be actively “made” and cannot just passively “happen.”
The rishonim discuss whether the direction in which one digs will make a difference in Rav Huna’s case. The Tosafot Ri”d and the Rosh, for example, argue that when enlarging the existing space so that it will be big enough for a sukka, one can only dig downwards, since all of the grain above the existing space has been established as the skhakh over the existing ohel. From the Ran, however, it appears that one can extend upwards, as well; because we have perceived it as skhakh all along, we view this as simply thinning it out rather than turning it into skhakh.