For me if their enclosure gets dirty it smells like the dark, wet areas of an old forest...sort of musty, sort of mushroomy if that makes any sense at all. It's not a bad smell and not at all like the strong urine smell a dirty rat cage would get. It definitely means though that you need to be tending that enclosure. They are very clean little critters but like any animal isn't not nice to leave them to live atop their own old feces and urine.