Mammoth Uncensored Word List:
77 words ending with um, 12 letters long

amphithecium androclinium appressorium archesporium baptisterium bimillennium calceamentum chlorophytum cladophyllum cladosporium crassamentum cuprammonium darmstadtium deinotherium diverticulum dolphinarium duraluminium epithalamium etymologicum gubernaculum helianthemum hibernaculum hypogastrium infundibulum inhalatorium interstitium involucellum kurchatovium leontopodium lithospermum mesogastrium mycodomatium neurocranium nonmagnesium nonpetroleum ornithogalum osteospermum paludamentum pandaemonium panhellenium perichaetium perinephrium perionychium periostracum periselenium pityrosporum pleiochasium postliminium praseodymium progymnasium protactinium prothalamium pseudopodium quadriennium quinquennium radiothorium receptaculum spermagonium spermogonium sporangiolum stomatodaeum suboperculum supernaculum superpremium superstratum suspensorium synclinorium transuranium understratum unguentarium unnilpentium unnilquadium unnilseptium verticillium verumontanum xiphisternum xylobalsamum