Yevamos, Chapter 16, Mishnah 4. This mishnah deals with a man who is suspected of having drowned, and with the question of when his wife may remarry. The basis of the discussion lies in a distinction between two types of bodies of water, and in a dispute concerning them.
Two types of bodies of water:
Water that has an end - a body of water whose every area and entire shoreline can be seen, so that if the man had come out of it, he would have been seen.
Water that has no end - a body of water whose entire shoreline we cannot see. Of course it has an end, but we cannot see it, and it is possible that the man came out of the water without our being aware of it.
In water that has no end, everyone agrees that the wife remains forbidden, since we cannot assume that he drowned - perhaps he came out and we did not see him. The dispute in our mishnah revolves around water that has an end, whose every edge can be seen: some hold that even there the wife is forbidden, and some hold that she is permitted, and we may assume that he drowned.
The opinion of the first Tanna:
"Nafal la'mayim, bein sheyesh lahen sof bein she'ein lahen sof - ishto asurah" - whether in water whose edges can be seen or in water whose edges cannot be seen, whether we would have been able to see him come out or not, his wife is forbidden. The reason: it is possible that the man remained in the water far longer than one would imagine, perhaps even several days. There is therefore no proof that he drowned merely because he stayed in the water long enough to have drowned.
Rabbi Meir's proof:
"Amar Rabbi Meir: ma'aseh be'echad shenafal la'bor ha'gadol, ve'alah le'achar sheloshah yamim" - a man fell into the large water cistern, and only after three days did he come up and out. This cistern is certainly a body of water that has an end, whose edges can be seen, and nevertheless he did not come out immediately. This shows that a person can survive in water for a long time, perhaps even more than three days, and this supports the opinion of the first Tanna that the wife is forbidden in every case, lest he is still alive.
The opinion of Rabbi Yosei:
"Amar Rabbi Yosei: ma'aseh be'suma sheyarad litbol bame'arah, veyarad moshcho acharav" - a blind man went down to immerse in a mikvah inside a cave, and the man who was leading him went down with him. "Veshahu sham kedei tum'as meis" - those standing there waited until enough time had passed that they could not have survived. "Vehisi'u es nesheihen" - they permitted their wives to remarry, since in such a place, had they come out, they would have been seen; and once they did not come out, we assume that they drowned and could not have survived in the water.
The incident in Asya:
"Veshuv ma'aseh be'Asya be'echad sheshilshelu'hu la'yam, velo alah beyadam ela raglo" - in a region called Asya they lowered a man into the water in the manner of a diver, with something tied to his leg. When they wanted to bring him up out of the water, all they brought up was his leg alone.
"Amru chachamim" - and this is how they ruled:
"Min ha'arkubah ulema'alah - tinasei" - if the leg that was found had been severed from the knee and above, his wife may remarry. For from the laws of tereifos in Maseches Chullin we learned that a person who lost his leg above the knee is a tereifah, and at least in the time of the Gemara he could not have survived such a loss.
"Min ha'arkubah ulematah - lo tinasei" - if the leg was severed below the knee, his wife may not remarry, since it is possible that he came out of the water and survived.
One point should be made precisely: even in the case where she was permitted, the permission takes effect only after twelve months have passed. A tereifah cannot survive more than twelve months, but that is no guarantee that he died before then. It emerges that the word "tinasei" stated here means only after twelve months.
This incident involved water that has no end, and nevertheless they were able to see all of its sides, and therefore even according to Rabbi Yosei the wife may remarry when the leg was found severed from the knee and above.
In summary: in this mishnah we learned the distinction between water that has an end and water that has no end; the opinion of the first Tanna, that in both cases his wife is forbidden out of concern that he survived in the water for a long time, and Rabbi Meir's proof from the man who came up out of the cistern after three days; and the opinion of Rabbi Yosei, who permits in a place where all the sides can be seen, as in the incident of the blind man and his guide and the incident in Asya, in which the ruling was decided according to the laws of tereifos, distinguishing between a severing from the knee and above and a severing from the knee and below.