Mammoth Uncensored Word List:
37 words ending with um, 13 letters long

acaridomatium acarodomatium actinouranium agrobacterium amphigastrium anticlinorium armamentarium chrysanthemum decamethonium evangeliarium ferrochromium heterophyllum hexamethonium hypochondrium inclinatorium lectisternium mycobacterium odontoglossum organolithium paleostriatum paramenstruum perichondrium platiniridium protoactinium protocerebrum pycnoconidium rutherfordium scolopendrium spathiphyllum spermatangium succinctorium sustentaculum tintinnabulum tritocerebrum vibracularium vivisectorium zoosporangium