They are also alarmed by the subsequent explosions caused by the foamy substance that have killed thousands of animals.

“This has all started in the last four or five years here. We don’t have any idea where it came from or how it got started,” agricultural engineer Charles Clanton of the University of Minnesota told Wired magazine. “Whatever has happened is new.”

The gelatinous foam traps methane, a flammable gas that can ignite and cause catastrophic explosions.

One such explosion last September leveled an entire barn in Iowa, killing about 1,500 pigs and injuring one worker.

Some of the blasts occurred as farmers poured water on the foam, or tried to break it up with machines.

A spark from an electric motor or a lit cigarette can also ignite the gas.

Researchers say the goop appears to be the byproduct of bacteria, but they don’t know which strain or where it might have come from.

Photos: Charles Clanton - University of Minnesota