Talmud for Nazir 9:2
נָזִיר שֶׁגִּלַּח וְנוֹדַע לוֹ שֶׁהוּא טָמֵא, אִם טֻמְאָה יְדוּעָה, סוֹתֵר. וְאִם טֻמְאַת הַתְּהוֹם, אֵינוֹ סוֹתֵר. אִם עַד שֶׁלֹּא גִלַּח, בֵּין כָּךְ וּבֵין כָּךְ סוֹתֵר. כֵּיצַד, יָרַד לִטְבֹּל בִּמְעָרָה וְנִמְצָא מֵת צָף עַל פִּי הַמְּעָרָה, טָמֵא. נִמְצָא מְשֻׁקָּע בְּקַרְקַע הַמְּעָרָה, יָרַד לְהָקֵר, טָהוֹר. לִטַּהֵר מִטֻּמְאַת מֵת, טָמֵא, שֶׁחֶזְקַת טָמֵא טָמֵא וְחֶזְקַת טָהוֹר טָהוֹר, שֶׁרַגְלַיִם לַדָּבָר:
If a Nazirite shaved, and then it became known to him that he had become unclean, [i.e., if he brought his offerings and shaved over them, and then he discovered that he had become unclean in the days of his Naziritism] — if it were known uncleanliness [uncleanliness that could be known of, as when it were not "buried in the depths"], it offsets, [and he counts another Naziritism.] And if it were "uncleanliness of the depths," it does not offset. [If it became known to him that in the place he had passed through there were "uncleanliness of the depths," i.e., uncleanliness which no one, even in the ends of the world, was aware of, even though it were definitely unclean, it does not offset, this being a halachah of Naziritism.] If [it became known to him] before he shaved, in either event, [whether it were known uncleanliness or uncleanliness of the depths], it offsets, [for the halachah that uncleanliness of the depths does not offset applies only after the shaving of cleanliness]. How so? [i.e., What is "uncleanliness of the depths"?] If he went down to immerse himself in (the waters of) a cave, and he found (matter from) a dead body floating on the surface of the cave, he is unclean. [If he were unclean with creeping-thing uncleanliness, or with something similar — not dead-body uncleanliness — and he went down to immerse (and cleanse himself) from his uncleanliness, and he found an olive-size (of matter) from a dead body floating on the surface of the water, and he were in doubt as to whether he had or had not become unclean, he is unclean. (As to the ruling that in the instance of uncleanliness floating on the surface of the water, he is clean, that refers to creeping-thing uncleanliness, but with dead-body uncleanliness, he is unclean.) And if this possibility became known to him after he had shaved, he is unclean. For this is known uncleanliness, since it was in a place where people would see it. As to its being stated: "If he went down to immerse," this is to strengthen the point, i.e., even though one who goes down to immerse from uncleanliness to cleanliness guards himself against all manner of uncleanliness, he is still unclean.] If it were found embedded in the soil of the cave [in the place where he immersed, so that he had become unclean of a certainty — if it became known to him after he had shaved, he is clean, and it does not offset (the Naziritism), this being "uncleanliness of the depths," it not having been known to anyone.], if he had gone down to cool off, he is clean. [Even if he had gone down to cool off, and not to immerse himself (in which instance he would take care to guard himself against uncleanliness), even so, he is clean], but (if he had gone down) to cleanse himself from dead-body uncleanliness, he is unclean. For a status of clean remains clean, and a status of unclean remains unclean. [If he went down to cleanse himself from dead-body uncleanliness, and immersed himself in a cave where a dead body was embedded, and he completed his Naziritism; or if he went down to cleanse himself from dead-body uncleanliness and then assumed Naziritism, he is unclean and offsets (his Naziritism). For a status of clean remains clean and a status of unclean remains unclean.] For there is a rationale for this, [for saying that the halachah that a Nazirite in the instance of "uncleanliness of the depths" is clean applies if he were in a status of clean, and not if he were in a status of unclean.]