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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. 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.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.