Mikvaos, Chapter 6, Mishnah 1. This mishnah takes up the halachos of joining two separate collections of water so that they are reckoned as a single mikveh, and how much of a connection between them is required.
The governing principle
The mishnah opens with a rule: "Kol hame'urav lemikveh, kemikveh." Anything blended together with the mikveh carries the mikveh's own status. Water that lies alongside the mikveh and is attached to it thereby acquires the halachah of that mikveh.
Water in the crevices of a cave
The mishnah illustrates: "Chorei hame'arah usedakei hame'arah, matbil bahen kemah shehen." Picture a cave with water collected inside it. The walls of that cave have holes and fissures, and water has settled into those little pockets too, touching the main pool that fills the cave. In such a case he may immerse in those pockets exactly as they are, even though only a tiny quantity of water sits in them and forty se'ah is certainly not to be found there. Since even the barest touching links that water to the mikveh, everything counts as one large mikveh, so the pocket is treated as though forty se'ah were present in it as well.
A hollow carved out on its own
The halachah shifts, though, when we come to "ukas hame'arah," a hollow that was dug out as a compartment of its own inside the cave, without any natural attachment to the water around it. Should a person want to link that hollow to the mikveh, then "ein matbilin bah, ela im kein haysah nekuvah keshfoferes hanod." Immersion in that hollow is invalid unless it has an opening as wide as the stopper of a wineskin, whose measure is an opening wide enough for two fingers to be turned around inside it comfortably, berevach, which we calculate today at roughly three inches.
Why the difference? The ukah is a genuinely independent unit, not simply a recess in the cave wall. Whatever is independent requires a substantial linkage in order to be attached, and substantial linkage here means keshfoferes hanod.
Rebbi Yehudah's qualification
The mishnah then records a further view. "Amar Rebbi Yehudah: eimasai?" In which circumstance do we insist that an independent hollow have an opening of a shfoferes hanod? "Bizman shehi ma'ameded es atzmah." Where the hollow supports itself. Meaning, the man excavated this space below, within the body of the cave, so that a natural roof remained over it, and that roof is firm enough that a man could stand upon it and it would bear his weight without giving way. Such a hollow is a self-standing chamber, genuinely separate from the water around it, and to attach it one requires the full measure of keshfoferes hanod, so that it becomes one mikveh with the water beside it.
"Aval im einah ma'ameded es atzmah, matbil bah bechol mah shehu." If, however, a man standing on the roof of this hollow would cause it to buckle and collapse, it is not viewed as a chamber in its own right whatsoever. Then the slightest measure of attachment to the mikveh will do; the hollow counts as belonging to the mikveh from the outset, and no opening of keshfoferes hanod is called for.
The mishnah thus draws a careful line: water tucked into the natural holes and cracks of the mikveh's own basin is simply part of the mikveh, while a space carved out as an independent chamber must earn its connection through an opening of the proper shiur, unless it is too flimsy to count as a chamber at all.