Just what the heck was that foam on the Des Plaines River on Monday? So far, no one seems to know.

But piles and piles of the foam – some several feet high – appeared on the river in the area around the Hofmann Dam. No one knows if the foam is the result of nature or something a bit more nefarious.

Allison Fore, director of public affairs for the Cook County Metropolitan Water Reclamation District said the Landmark’s inquiry about the phenomenon sparked an investigation by the agency.

The result of that investigation was pending Monday afternoon. A call to the Illinois Department of Natural Resources was not immediately returned.

Terry Keating, a Riverside resident who lives near the river, said he noticed foam mounds up to 6 feet high by the dam and other smaller foam patches scattered downriver.

“It was just churning,” Keating said. “The fishermen were dumbfounded. They were just packing it up.”

Riverside Village Manager Peter Scalera also witnessed the foam but had no explanation for its appearance.

Neither Keating nor Scalera noticed any odor associated with the foam, although one person who was at the scene, Brookfield resident Debbie Zari, who works for the Landmark’s parent company Wednesday Journal, said there was a slight chemical smell.

Fore reminded anyone who sees anyone dumping anything into the river or sees anything odd on the waterway to contact the MWRD hotline at 1-800-332-DUMP.

“If you see anyone dumping anything into the river let us know,” Fore said, “because it could be dangerous and could be a criminal act.”