I located the source of the odor in our basement. For the last two weeks, I thought it was due to the mold that flourished after our tenant's hot water heater ruptured. No amount of bleach seemed to clear away the stench.
Then, last night, T was collecting the laundry when she noticed a bizarre, highly viscous drizzle from some insulation in the ceiling. She told me about the drizzle (it had the approximate color and flow of rubber cement), and I went down into the basement for what I realized would be a nasty job. I put on a mask and gloves and pulled the insulation down.
I now know the meaning of the word "miasma;" I feel what it means in the center of my being.
