Mikvaot, Chapter Six, Mishnah One
1) Anything which is joined with [the water of] a mikveh is like a mikveh.
2) One may immerse in holes of a cavern and in crevices of a cavern just as they are.
3) The pit of a cavern, one may not immerse in it unless it had a hole as big as the tube of a water-skin.
4) Rabbi Judah said: when [is this the case]? When it stands by itself; but if it does not stand by itself, one may immerse in it just as it is.
Section one: A small body of water which is joined with a valid mikveh takes on the properties of the mikveh and can be used as such, even if it doesn’t contain forty seahs.
Section two: This is an example of the principle in section one. One may use the holes in the sides of a cavern in which a mikveh is found, even if they are not connected to the mikveh itsel by a hole the size of the tube of a water-skin.
Section three: This pit is dug out next to the mikveh in the cavern and is not part of the cavern itself. For this pit to be used as part of the mikveh it must be attached to the mikveh by a hole at least as wide as the tube of a water-skin. This same measure was mentioned in 4:5 concerning attaching a trough to a mikveh.
Section four: Rabbi Judah limits this to a case when the pit stands on its own. If the pit does not stand on its own, but rather is leaning on the walls of the mikveh, then it is considered part of the mikveh and one can immerse in it even if it is not attached by a hole the size of the tube of a water-skin.