Mikvaot, Chapter Six, Mishnah Seven
Our mishnah talks about how two mikvaot can be joined together such that if each has less than forty seahs of water, we can consider them both together so that they have forty seahs.
1) Mikvaot can be joined together [if their connection is as big] as the tube of a water-skin in thickness and in space, in which two fingers can be fully turned round.
2) If there is a doubt [whether it is as big] as the tube of a water skin or not, it is invalid, because [this is a mitzvah] from the Torah.
3) The same applies also to the olive’s bulk of a corpse and the olive’s bulk of carrion and the lentil’s bulk of a sheretz.
4) Anything which remains in [the space measuring] the tube of a water-skin lessens [its measure].
a) Rabban Shimon ben Gamaliel says: if it is any water creature whatsoever, [the mikvaot] remain clean.
Section one: The hole connecting the two mikvaot must be the size the tube of a water-skin, which is two fingerbreadths in width. As the mishnah explains, one must be able to put one’s fingers in the tube and fully turn them around.
Section two: If someone immerses in one of these mikvaot, and it alone has less than forty handbreadths and he is not sure whether the connection with the other mikveh is as big as the tube of a water-skin, he remains impure. This is because the mitzvah to immerse in the mikveh is from the Torah and in cases of doubt concerning toraitic impurity, the law is strict.
Section three: If there is a doubt whether a piece of corpse is the bulk of an olive, which is the size required to defile, or a piece of carrion (of an impure animal) is the size of an olive or a dead sheretz (creepy-crawly thing) is the size of a lentil, in all of these cases the law is stringent and that which it touched is considered impure. Again, this type of defilement is from the Torah.
Section four: If something remains in the hole that connects the two mikvaot, it reduces the size of the hole. If it is less than the size of a water-tube, the two mikvaot are not considered connected.
Rabban Shimon ben Gamaliel says that if there is some sort of water-creature, perhaps a water-worm of some sorts, in the hole, it does not reduce the size of the hole. If someone immerses, he will be clean.