We found the source of the smell (NSFDinner)

A peculiar smell filled the sinister expanse of our basement. We loaded the Victorian-era sinks to clear anything stuck in the drainage traps, disposed of old paints and chemicals, and inspected the closets and boxes for culprits. Nothing.

Weeks passed, until the odor began to permeate upstairs into the house itself. Leaving no corner unexplored, we finally lifted the toilet lid in the basement's ancient, haunted lavatory. Floating, face down, was a massive drowned rat. Underwater, the fur remained; in the air, it had delaminated. An island of smooth, naked ratskin rose from the water, bordered by a grayish crown of putrefaction.

Nearby, the remains of a fly.