Mikvaot, Chapter Four, Mishnah Five
Today’s mishnah deals with the water gathered either in a trough found inside a rock, or with some sort of containing vessel made out of the stone of such a trough.
1) In the case of a trough in a rock:
a) One may not fill up [the hatat waters] from it, nor may the [hatat waters] be consecrated in it, nor may one sprinkle from it.
b) And it does not require a tightly stopped-up covering,
c) And it does not invalidate the mikveh.
2) If it was a vessel and had been joined to the ground with lime:
a) One may fill up the hatat waters from it and the hatat waters may be consecrated in it, and one may sprinkle from it,
b) And it requires a tightly stopped-up covering;
c) And it invalidates the mikveh invalid.
3) If a hole was made in it below or at the side so that it could not contain water in however small a quantity, it is valid.
a) And how large must the hole be? Like the tube of a water-skin.
b) Rabbi Yehudah ben Batera said: it happened in the case of the trough of Yehu in Jerusalem that there was a hole in it like the tube of a water-skin, all the pure things in Jerusalem were made using it.
c) But Bet Shammai sent and broke it down, for Bet Shammai say: [it remains a vessel] unless the greater part of it is broken down.
Section one: While the trough is still in the rock, it doesn’t count as a vessel. There are three areas of consequence to this. The first is that it can’t be used for various parts of the preparation of the “hatat waters” those waters into which the ashes of the red heifer are mixed. This same mishnah can be found in Parah 5:7.
Secondly, since it is not a vessel it does not need a tightly-fitting cover to protect its contents from becoming impure if found in a tent (an ohel) with a dead body.
Third, the water in it is not considered drawn water such that it doesn’t disqualify a mikveh.
Section two: If he cut the trough out of the rock and made a vessel out of it, all of the opposite is true. Now it is a vessel so it can be used for the hatat waters but not for the mikveh.
Section three: A trough can be nullified from being a vessel by having a hole in it the size of the tube of a water-skin. This is about two fingers in width. Since it can no longer contain water, it is no longer considered a vessel.
Rabbi Yehudah ben Batera relates a story that happened during the Second Temple period in Jerusalem. The water in this trough was used to in the mikvaot of Jerusalem and everything that went through those mikvaot was considered pure.
Until Bet Shammai came along, that is. Bet Shammai holds that the trough remains a vessel until the greater part is broken. So Bet Shammai actually sent for the trough and had it broken down to accommodate their purity stringencies. The mishnah does not say how Bet Hillel responded, but I am sure they were not happy.