Mishnah for Nazir 9:3

If he was searching and came to a river bed, or a pool or a public road, he may end [his search]. A field in which men have been slain, he may gather the bones one by one, and all [the area] may be accounted clean. One who removes a grave from his field, he may gather the bones one by one, and all [the area] may be accounted clean. A pit into which they throw fetuses or people that had been slain, he may gather the bones one by one, and all [the area] may be accounted clean. Rabbi Shimon says: if it had been prepared as a grave in the first place, there is [blood-] saturated earth.