Nazir 63 - March 27, 5 Nissan

If a nazir finds out after shaving that he had become impure before, the earlier days are canceled and the nezirut must be redone, but only if the impurity was known. However, if it was tumat tehom, then nothing is canceled. Tumat tehom is if one went to a mikveh in a cave and there was a dead body that sunk to the bottom and could not be seen. If one went into the mikveh to cool off and did not see the body, one would not be considered impure. But if one used it as a mikveh, it would not be effective as the person's presumptive status stays the same - if one was impure, they remain impure. What is the source for tumat tehom? Reish Lakish and Rabbi Elazar bring verses, but after raising two questions, one on Rabbi Elazar and another on both, they conclude that it is a halacha l'Moshe m'Sinai. The Mishna follows Rabbi Eliezer who holds that shaving the hair is necessary for finishing nezirut, as that is the determining factor for tumat tehom. Rami bar Hama asked: If one became impure during their nazirite term, but only found out after the term ended, before they shaved, does it cancel thirty days, as it goes by the day they became impure, or does it only cancel seven? After some back-and-forth answers and difficulties, they derive the answer from the Mishna. In Tosefta Zavim 2:5, they define what are cases of tumat tehom.