My basement is segmented off into five chambers: the movie room, an office, a closet, the furnace room, and the sump pump room. Every great once in a while I'd get a faint urine smell down there, and would immediately scold my teenager for not doing a good job cleaning the cat box. Fast forward to earlier this week, we came home from being gone for 3-4 days and the basement had a STRONG smell of urine. Once again, I kicked the teenager in the ass and she emptied the cat box. Thing was, neither the cat box nor the furnace room in which it sits smelled at all! The urine smell was coming from somewhere else... So I did a perimeter check of the basement on my hands and knees, feeling and smelling for wet spots. Maybe the cat had started scenting? Nothing. It was at that moment that I put the puzzle together: 1) The previous owners had gotten mice in the sump pump room a few years ago. 2) The mice were gone, but as mice do they had pissed all over the concrete floor. 3) We've just had a record amount of rainfall. And while the basement has been bone dry, the sump pump has been working overtime, introducing a decent amount of humidity into that space... Cliff Notes: During heavy rain periods, the sump creates a lot of moisture in that room, coming into contact with the dried mouse urine, causing it to "activate" and stink up my basement. Bleach + brush + shop vac = smell's gone!