Ohalos, Chapter 9, Mishnah 11. This chapter follows a kaveres, a large beehive, through one situation after another, asking a single question each time: does this kaveres serve as an ohel that blocks tumah from passing through it, or is it simply a keli, a vessel, which cannot hold tumah back? Our mishnah takes the case of a kaveres that is lying on its side out in the open air.
The mishnah begins: "Hoysah mutah al tzidah ba'avir" - the kaveres has been tipped over, so that it lies along its side instead of resting on its base. It sits on the ground, and the word ba'avir tells us where: in the open air. Imagine standing next to it and raising your eyes; there would be nothing above you but sky. No roofing covers it, no ohel of a building spreads over it, and so the entire body of halachos governing how tumah travels within a house has no place in this mishnah.
Now the mishnah introduces the tumah: "ukezayis min hameis nosun tachtehah oy al gabah" - a kezayis from a meis was placed either beneath the kaveres or on top of it. What happens to the vessels in the area?
In line with the tumah
"Kol shehu keneged hazayis, tachtehah ve'al gabah, tamei" - anything that lies in a direct line with that kezayis, whether the tumah is below and the keli is above it, or the tumah is above and the keli is below it, becomes tomey. Why? Because this is a working, functioning kaveres, and a functioning kaveres has the status of a keli. A keli does not shield anything from tumah; it is no barrier. So the tumah rises straight up or descends straight down through the kaveres as if the kaveres were not there, and whatever stands in that column becomes tomey.
Off to the side, and inside
"Uvechol she'eino keneged hazayis, toichah tahor" - whatever is not directly in line with the kezayis remains tohor. If the kezayis sits on top and a keli lies underneath but shifted a bit off to the side, it stays tohor; and the same if the tumah is beneath and the keli above it is not truly in the straight line. And toichah, the inside of the kaveres itself: a keli sitting within the kaveres stays tohor even when it is standing squarely in the path of the kezayis.
Here we see the two halves of this kaveres's character. It is so large that it is not susceptible to tumah at all, and therefore it protects whatever is placed inside it. That protection holds whether the tumah above or below happens to be in a direct line with the inner keli or not. But at the same time, precisely because it is a functioning keli and not an ohel, it cannot stop tumah from travelling through it. That is why tumah beneath it reaches a keli resting on top of it, and tumah on top of it reaches a keli lying beneath it.
When the tumah is inside
"Betoichah, hakol tamei" - should the kezayis from a meis be placed inside the kaveres, every keli in the picture becomes tomey. Every keli means just that: one sitting in the interior beside the tumah, one resting on the kaveres from above, and one lying on the ground beneath it. The reason is that once tumah occupies the hollow, the whole inner space of the kaveres counts as filled with tumah, and a kaveres that is no ohel holds nothing back. That entire filled space therefore sends tumah up in a column and down in a column, so that any keli caught above or below is tomey, and at this stage it makes no difference whatsoever whether it stands opposite the kezayis or off to one side.
To review the mishnah as a whole: our kaveres is lying on its side in a courtyard or a garden, with no ohel over it and no house involved, only open air. When the kezayis of a meis is outside the kaveres, above it or below it, it conveys tumah to whatever lies in a straight line on the other side, since a functioning keli forms no barrier. What is inside the kaveres, however, is spared, because a vessel of this size is not susceptible to tumah and shields its contents. But once the kezayis is placed inside the kaveres, the interior is flooded with tumah, and everything inside, above and below turns tomey.