[If] a man fell into water, whether all its shores are in sight or not, his wife is prohibited. R' Meir said: There was [once] an incident where someone fell into a large well and surfaced after three days. R' Yose said: There was [once] an incident where a blind man descended into a cave to immerse himself, and was followed by his guide; they remained [undetected] long enough from them to have died, and [the Rabbis] permitted their wives to remarry. There was another incident at Asya, where someone was lowered into the sea and they recovered nothing but his leg. The Sages said: [If the leg was severed] above the knee, she may remarry; [if the leg was] below the knee, she may not remarry.