Coffins which may contain adequate airspace so that impurity does not exit the coffin: Berachot 19b
Stepping over a grave: Berachot 19b
A Kohen who leaned over a pit, in which a corpse may lie: Pesachim 9a; Niddah 15b
Hillocks near a city or road: Ketuvot 20b
New Hillocks far from a city or road: Ketuvot 20b
Old Hillocks far from a city or road: Ketuvot 20b
Definition of "City," "Road," "Near," and "Far": Ketuvot 20b
Possible Cemeteries in Israel: Ketuvot 20b