A strange new growth has emerged from the manure pits of midwestern hog farms. The results are literally explosive.

Since 2009, six farms have blown up after methane trapped in an unidentified, pit-topping foam caught a spark. In the afflicted region, the foam is found in roughly 1 in 4 hog farms.

There's nothing farmers can do except be very careful. Researchers aren't even sure what the foam is.

"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," said agricultural engineer Charles Clanton of the University of Minnesota. "Whatever has happened is new."

A gelatinous goop that resembles melted brown Nerf, the foam captures gases emitted by bacteria living in manure, which on industrial farms gathers in pits beneath barns that may contain several thousand animals.

Foam covers a manure pit. Photo: Charles Clanton

The pits are emptied each fall, after which waste builds up again, turning them into something like giant stomachs: dark, oxygen-starved percolators in which bacteria and single-celled organisms metabolize the muck.

Methane is a natural byproduct, and is typically dispersed by fans before it reaches explosive levels. But inside the foam's bubbles, methane reaches levels of 60 to 70 percent, or more than four times what's considered dangerous. The foam can reach depths of more than four feet.

Disturb the bubbles, and enormous quantities of methane are released in a very short time. Add a spark – from, say, a bit of routine metal repair, as happened in a September 2011 accident that killed 1,500 hogs and injured a worker – and the barn will blow.

If it's easy to see what the foam causes, however, it's much more difficult to understand what causes the foam.

Among the possibilities are new bacterial communities that cause foam to form, or a change to the molecular structure of hog waste – a new foodstock, for example, or a pit-cleaning soap that makes the waste more frothy.

Or it could be both factors, or neither. Scientists have so far been stumped by the foam's patterns.

It can appear in one barn but not another on a farm where every barn is operated identically. Once the foam's established, it keeps coming back, regardless of efforts at cleaning and decontamination.

But though it's now common in southern Minnesota and northern Iowa, and in adjacent parts of northwestern Illinois and southwestern Wisconsin, the foam doesn't seem to be spreading outside that area.

A possible clue comes from historical experiences at wastewater treatment plants, where similar-looking foams have been caused by bacteria, though the identified species can't always survive in low-oxygen environments like manure pits.

If microbes are to blame, the next question would be: Why now? Deep-pit manure collection on high-density hog farms has been around for decades. Some recent and specific change would need to be responsible for altering the communities of microbes inside them.

"I don't think it's a dangerous new microbe," said Angela Kent, a microbial ecologist at the University of Illinois at Urbana-Champaign. "I think it's a shift in the environment that's favoring a particular microbial assemblage that's inadvertently causing this."

One possibility is a dramatic rise in the agricultural use of so-called distiller's grain, a byproduct of alcohol and ethanol production: Between 2001 and 2003, the amount of distiller's grain in hog food quadrupled in the United States. Some evidence suggests a link to foaming, though it's still tentative.

Changes in water use, antibiotic distribution and even corn genetics have also been suggested as hypothetically plausible culprits, but hypothetical is the operative term.

Kent is current comparing microbial differences between foam and foam-free manure pits, and hopes that a new round of carefully controlled studies on farms using pigs with identical characteristics and diets will give new insight into this unlikely scientific frontier.

"I don't think anyone's very familiar with what microbes are present in a manure pit on a hog farm," she said.