2,906. What Is an Unknown Source of Impurity?

Hilchos Nezirus 6:18

“Unknown impurity” (in the previous two halachos – literally “impurity of the depths”) means that no one anywhere in the world knows about it. This only applies to a person who died naturally and not to a murder victim. This is because, in the latter case, the murderer would know about it.

Hilchos Nezirus 6:19

If a corpse is found in the open, it’s not considered unknown impurity; if it’s found sunken in a cave and covered with water, that’s a case of unknown impurity. If a body is found buried in straw or pebbles, it’s unknown impurity, but if it’s found underwater in the dark or in a crevasse, it’s not.