IF HE DEPOSITED [THE ERUV MEAL] ON A TREE ABOVE [A HEIGHT] OF TEN HANDBREADTHS, HIS ERUV IS INEFFECTIVE; . . .
IF HE DEPOSITED IT IN A CISTERN, EVEN IF IT IS A HUNDRED CUBITS DEEP, HIS ERUV IS EFFECTIVE.
After discussing heights, like walls and treetops, we now gaze into the depths.
Where was this CISTERN situated? If it be suggested that it was situated in a private domain, is [not this ruling, it may be objected,] obvious, seeing that a private domain rises up to the sky, and as it rises upwards so it descends downwards?
If, on the other hand, it be suggested that it was situated in a public domain, where [it may again be objected] did the man intend to have his Sabbath abode? If above, he would be in one domain and his eruv in another; and if below, [is not the ruling again] obvious seeing that he and his eruv are in the same place?
It is decided that the Mishnah related to a cistern in a karmelet – an “in between” space, not quite public and not quite private (i.e. and area in a public way, but which few people travel on).
In any case, it is interesting that “up” (because of the use of trees or other items) is more complicated than “down.”