Ohalos, Chapter 15, Mishnah 5. Our chapter has been dealing with partitions and coverings inside a house and how they divide one space from another for the laws of tumah. This mishnah turns to a partition of a very particular kind: not a wall standing upright, and not a ceiling overhead, but a second floor laid over the natural ground of the house.
The case of the mishnah
The mishnah opens: "chatzatzo me'artzo," he separated off his house from its ground. What is being described? The house has its karka, its natural earthen floor. Over that karka the owner laid boards, or spread out curtains and hangings (yerios), so that now there are two layers of flooring with a bit of empty space trapped between them. This is not a house of two stories. It is one room, with two levels of floor at the bottom and a gap in between. How large that gap must be is exactly what the mishnah will come to define.
When the tumah is in the gap
"Tumah b'chatzatz, keilim sheb'bayis temei'im." Should the source of tumah be lying in that hollow underneath the upper flooring, every kli standing in the room overhead becomes tamei. The mishnah is picturing a hollow that is sealed off on every other side, so that the tumah has nowhere to travel except upward. Because its one available exit leads through the room, the room and its contents catch the tumah on the way out.
Tumah in the room above
"Tumah b'bayis, keilim sheb'chatzatz." Turn the case around: the source of tumah is up in the room, while the keilim are resting in the hollow underneath. Now everything hinges on the dimensions of that hollow. The mishnah rules "im yesh bimkomo" a tefach in length by a tefach in width, "al rum tefach," with a full tefach of height as well, the keilim there stay tahor. What makes this direction of the case more lenient? A source of tumah sitting in the room is not dependent on the hollow at all for its exit; it departs by way of the door or a window, as tumah does in any ordinary house. Consequently the only thing needed to save the keilim underneath is that their space stand on its own as an ohel: a tefach by a tefach, with a tefach of clearance. The moment the hollow qualifies as an ohel, it is a reshus by itself and shelters whatever is inside it.
The parallel from Perek Gimmel
The Bartenura points us back to a case taught in the third perek, mishnah zayin: a roofed conduit passing under a house. Inside, the conduit had the width of a tefach, but at its outlet, where the water ran off, it lacked an opening of a tefach. The halacha there followed precisely the pattern before us now. Tumah in the house left whatever sat inside the conduit tahor, for such tumah has its own route of departure. Tumah inside the conduit, however, made everything in the house tamei, since the house was its only path out. Our mishnah takes that same reasoning and applies it to a room whose flooring has been laid in two levels: tumah in the hollow is compelled to rise and pass through, so the vessels above contract tumah; tumah in the room has no use for the hollow, so the vessels below can be spared.
When the space is too small
The mishnah continues: "v'im lav, temei'im." If the space between the two floor levels does not have the shiur of a tefach by a tefach by a tefach in height, the keilim lying there are tamei, even though the tumah in the house had no need of that space at all. And the mishnah gives the reason: "she'artzo shel bayis k'moyo ad hatehom," the ground of the house is like the house itself all the way down to the depths.
This is a fundamental principle. As a rule, tumah found in a house descends, and it renders tamei whatever lies beneath the house down to the tehom. The only thing that interrupts that descent is a genuine ohel: a space of a tefach in length, a tefach in width and a tefach in height, which stands as a separate domain. Absent such a space, whatever lies under the house is simply part of the house. It is no different from an object stashed away somewhere inside the room.
The Torah teaches this in the posuk, "V'chol asher ba'ohel yitma shivas yamim," and all that is in the tent shall be tamei seven days. The all-inclusive language of the posuk sweeps in the karka on which the room stands, together with anything lodged beneath that karka; all of it counts as being inside the ohel. The one escape available to keilim concealed down there is that they occupy an ohel of their own, a tefach in each direction with a tefach of height.
The contrast with an ordinary partition
Notice carefully how this differs from a case of an ordinary mechitzah. Keilim placed on the far side of a partition inside a house, or even keilim resting on top of a partition that lies horizontally, do not need any ohel of their own in order to be tahor. The mechitzah itself does the work. It places those keilim outside the house, exactly as keilim on the roof of a house are tahor, or keilim standing outside the walls of the house are tahor. A partition blocks, and that is enough.
Keilim tucked into the earth underneath the room, the case of our mishnah, follow a different rule. The presence of a layer of boards spread above them is not by itself enough to take them out of the house. They emerge as a separate reshus, and remain tahor, only when they occupy a proper ohel of their own: a tefach by a tefach, with a tefach of height. Failing that, the tumah of the room descends without anything to halt it. Someone who buries his possessions in the ground of his room for safekeeping has, in the eyes of the halacha, simply left them in the room.