The Mishnah taught that any pool of water that mingles with the water of a mikveh is as valid as the mikveh itself. The Mishnah taught that one may immerse in holes and crevices of a cavern just as they are, but one may not immerse in the pit of a cavern except if it has an opening as big as the tube of a water skin. Rabbi Judah taught that this is the case when it stands by itself (and forms an independent pool separated by a wall from the pool in the cavern), but if it does not stand by itself, one may immerse in it just as it is (for it is part of the pool in the cavern).