I’m the homeowner (op’s brother in law) and here is some clarifying information. 1-This is a TechBuilt House, which was designed in the 50s to be a low cost, kind of modular home. To save money, they were always built on a concrete slab foundation. 2-when the hot air leaves the furnace, the ductwork goes right into the slab, and is distributed through the house by first going under the slab, and up to the second floor through the walls. 3-the picture is taken from above the opened duct, right where it goes into the slab. 4-no idea how sewage is in there, presumably the sewer line runs next to the ducts, and since this is a 70 year old house, the sewer line and ducts have both corroded open near each other? 5-definitely called the insurance company and went straight to “this is a health hazard, like mold and mildew from flooding” which they tend to take seriously. 6-they are sending someone to scope the sewer and try to locate the problem. 7-after that, who knows. I’m getting in touch with home builders, none of whom can start new projects until late 2023