Ohalos, Chapter 9, Mishnah 13. This chapter follows a kaveras, a beehive, through a series of positions and asks a single question each time: how far does tumah travel, and what does the kaveras itself shield? Until now the chapter has dealt largely with a kaveras lying on its side. Our mishnah changes the picture: the kaveras is outdoors, and it is standing upright, resting on its base, with its opening facing upward.
The case
"Haysah yosheves al shuleha": the kaveras is sitting on its bottom, so that its floor is on the ground and its mouth faces the sky. "V'keli": it is a whole, functioning vessel. The mishnah has to tell us this, because the following mishnah will take up the case of a kaveras that is no longer a proper keli. Since it is a keli, it has no power to stop tumah; a keli does not serve as a barrier.
The ruling: "tumah tachteha, b'socha, oy al gaba", if a k'zayis from a meis is found below the kaveras, within its hollow, or above it, then "tumah boykas v'oylah, boykas v'yoredes", the tumah splits through on its way up and splits through on its way down. Note the verb the mishnah selects: bokeia, from the very root used for the tearing open of the Yam Suf. Nothing standing in the path of this tumah holds it back; it drives straight through.
Why the language of breaking through
Consider what we had until now. When the kaveras lay on its side, tumah underneath it would rise in a straight line and render tamei whatever was directly above the kaveras, and tumah above it would descend in a straight line and render tamei whatever lay directly beneath it, yet the space inside the kaveras remained protected. Why? Because a kaveras lying on its side has, in effect, a wall above the hollow and a wall below it. Its interior was a domain of its own, sealed off from the column of tumah passing above and below.
Standing upright, the kaveras has no such protection. Its mouth gapes open toward the sky, so the interior is not a closed space at all. That is why the mishnah speaks of tumah breaking through. Tumah lying beneath the kaveras rises in a straight line and defiles everything it passes on the way up: the inside of the kaveras and whatever rests on top of it. Tumah resting above descends in a straight line and defiles the interior and then continues on to whatever lies beneath the kaveras. In the earlier case the rising and descending tumah skipped over the hollow; here it passes right through it.
So the Bartenura explains: the kaveras standing open is not an ohel unto itself, and it therefore has no ability to hold tumah back. This is true not only when the tumah sits on top, but equally when it lies underneath. For a kaveras to guard its own interior, it needs to be closed off both above and below, and a kaveras whose mouth faces upward is closed off neither above nor below.
One point of clarification. When the mishnah speaks of tumah on top of an open kaveras, how can that be? Anything placed there would simply drop inside. The case is tumah suspended over the mouth of the kaveras, hovering above the opening.
When the tumah spreads to the sides
Everything discussed so far has involved a narrow vertical shaft of tumah: only objects lined up directly with the tumah, above it or below it, are affected. The mishnah now introduces situations in which the tumah refuses to stay in that shaft and instead saturates the entire area the kaveras occupies, side to side. What creates that difference is the presence of an ohel.
So when does an ohel come into being here? The mishnah gives three possibilities. The first is "haysah gevohah min ha'aretz tefach", where a gap of a tefach separates the underside of the upright kaveras from the floor beneath it. That gap is itself a tent one tefach high, and within an ohel tumah travels in every direction. The second is "oy mechusah", a cover is placed over the mouth of the kaveras, giving it a roof. The third is "oy k'fuyah al piha", the kaveras is inverted so that it rests on its opening, and its base, formerly the floor, now functions as the roof above. The lesson is that clearance underneath is not the only way; a roof overhead achieves precisely the same result, and it makes no difference whether that roof is a separate cover or the kaveras's own bottom turned skyward.
In any of these cases: "tumah tachteha, b'socha, oy al gaba, hakol tamei", whether the tumah is beneath the kaveras, inside it, or above it, everything is tamei. Not merely what stands in a direct line with the tumah, but everything within the full span of the kaveras, because the ohel carries the tumah sideways. Whichever of the three positions the tumah occupies, it takes over the kaveras entirely.
Here a question presents itself. If the tumah is above and there is a lid in place, or the kaveras is overturned so that its base faces upward, should that covering not block the tumah from entering? That is precisely the difficulty, and precisely the point. Because the kaveras is a keli, it cannot block tumah, but it can spread it. A covering that is useless as a barrier is nonetheless effective as the roof of an ohel.
So the two halves of the mishnah fit together. An open, upright kaveras resting on the ground is no ohel at all, and the tumah simply drives straight through it, upward or downward. But give it a roof above, whether a lid or its own overturned base, or give it a tefach of clearance below, and it becomes an ohel. Then the tumah no longer travels in a narrow line: it fills the ohel, and everything inside the kaveras, above it, or beneath it, within its length and its width, becomes tamei.