Complex sentence: In Judaism, contact with a corpse confers uncleanness (see Numbers 19:11-22 and Tractate Oholoth in the Mishna), an unmarked grave opens up the possibility that a pious Jew could become defiled without being aware that it happened.
Split into simpler sentences: In Judaism, contact with a corpse confers uncleanness (see Numbers 19:11-22 and Tractate Oholoth in the Mishna). Thus, an unmarked grave opens up the possibility that a pious Jew could become defiled without being aware that it happened.