Ohalos, Chapter 9, Mishnah 8. Our chapter keeps turning the same case over and over, examining a kaveres (a beehive) that stands in the doorway of a house, half in and half out. Each mishnah changes one of the variables: is the kaveres still a functioning keli or not, does its opening face out toward the courtyard or in toward the house, and is it lying flat on the ground or raised a tefach above it. With every shift of a single detail, the ruling shifts along with it. Here the cycle comes around again, and our mishnah is built on the very same foundation as Mishnah 4.
The case before us. The kaveres is no longer a kaveres in the functional sense: it is not a keli. Since it cannot receive tumah as a vessel, it automatically serves as an ohel, and an ohel is something that blocks tumah and holds it in place. Its opening faces inward, toward the interior of the house, and not outward. And now the mishnah adds the new element: "hoysa gevoho min ho'aretz tefach," it is raised a tefach off the ground. That gap of a tefach is decisive, because a space of a tefach forms an ohel of its own underneath the kaveres, and an ohel spreads tumah throughout everything it covers.
The ruling. Three placements are listed. "Tumah tachteho": an olive's bulk of corpse matter resting in the space below the hive, that is, below the section jutting out into the courtyard. "O ba bayis": it lies anywhere at all within the house, its precise spot being of no consequence. "O besochah": it has been set down inside the hollow of the kaveres itself. In each of these three situations the verdict is "hakol tamei," every part of the picture is rendered tamei. Only one item is excluded, "ela gabo": a vessel sitting on the hive's outer upper surface.
Why everything joins together. Follow the two openings. Because the kaveres is raised a tefach, tumah placed underneath it does not stay put in a small pocket. The tefach of space beneath it is an ohel, so the tumah fills the entire area under the kaveres, and from there it continues on into the house and fills the house as well. And because the mouth of the kaveres is turned inward, the interior of the kaveres and the interior of the house are effectively one domain. Therefore tumah in the house rises into the inside of the kaveres and renders it tamei, and tumah inside the kaveres travels out through the opening and renders the house tamei. Underneath, inside, and the house itself are all bound together as a single ohel.
The contrast with Mishnah 4. There we dealt with a kaveres whose opening faced outward. Since the mouth was turned away from the house, the interior of the kaveres was cut off from what was going on inside, and the interior and its top could remain tahor. Here the opening faces the other direction, into the house, and that single reversal is what draws the inside of the kaveres into the house's tumah.
When the tumah lies on the hive's back. The exclusion just noted is now spelled out as the mishnah's closing case. "Al gabo": if an olive's bulk of tumah is resting on the upper surface of the hive, out on the courtyard side, then "kenegdo ad harakia tamei," the column of space rising straight over it, all the way to the heavens, is tamei, since nothing intervenes to hold it back. Outside that column, all remains tahor. The area below is untouched, for the hive, lacking the status of a keli, functions as an ohel and bars any downward spread. The house also stays tahor: with the mouth turned toward the interior, tumah resting on the outer surface has no route by which to enter. It simply ascends in a straight line and reaches nothing else.
So the picture of this mishnah is clear: a kaveres that is not a keli, raised a tefach, with its mouth turned toward the house, unites underneath, inside, and the house into one ohel of tumah, and leaves only its outer back excluded, where tumah rises straight upward and no further.