The hard, sticky substance that accumulates in sewers and causes 22 percent of overflows actually appears to be a type of soap formed by chemical reactions in the pipes, according to a research study reported in USA Today.

Environmental engineers at North Carolina State University found that the grayish-white  formations on the walls of sewer pipes are actually fatty acid salts – the chemical definition of a soap. “We're creating soap in the sewers but it's not something you'd want to wash your face with," said Joel Ducoste, a professor of environmental science at the university, in an interview with the newspaper.

Ducoste is a co-authors of a paper on the topic published in the journal Environmental Science and Technology. “Household soaps are made from either sodium or potassium mixed with fat,” USA Today reported. “But the researchers discovered that calcium can also mix with fat to create a fatty acid salt...and that's exactly what's happening in the sewers.”

The scientists still don’t know where the calcium is coming from that turns the FOG into chemical soap.