Jack Fisk knew he had a huge problem on his hands last Wednesday when a bale of hay in his barn that was smoldering from spontaneous combustion suddenly ignited "just like a bomb going off" as he tried to remove it with a skid loader.

Fanned by a steady breeze from the northwest, other bales of hay stacked three high started to burn as flames leaped as they do a forest fire -- and the barn held several hundred bales.

For the next 12 hours, Jack and at least eight fire departments near Sand Lake, Mich. fought to control the inferno. Smoke could be seen more than 10 miles away by motorists driving along US-131 who then called 911 to report the incident.

Water used to control the blaze proved ineffective, sometimes even making the situation worse. Moisture and high humidity cause spontaneous combustion of hay and other organic materials in the first place.

"All you can do is let a hay fire like this burn sometimes," says Jack, 46, who with his wife, Kacie, run the Fisk Dairy Farm started by his father, Ross, in 1947. "The more water you put on it, and it starts to burn even more days later." The fire departments used foam to contain the fire, but it's three days later and the hay still continues to smolder, possibly to catch fire again when the wind whips up.

So what really was responsible for the blaze that destroyed a hoop barn, several pieces of farm equipment and nearly 600 bales of alfalfa hay that Jack uses to feed his herd of 200 cows?

Turns out that plant respiration and the growth of molds and micro-organisms work in concert to bring the hay to the point of ignition in a range 180° to 210°F, according to a report from Lester Vough, a specialist with the University of Maryland. Respiration is the process of converting sugars and starches with the help of oxygen in the air into water and carbon dioxide, releasing heat in the process. Alfalfa continues this respiration even after it is cut, with the process winding down as the moisture content falls. Mold and bacteria also generate heat as they grow in a moist environment.

Jack says he is insured for the damage, but the fire has set him back on other work that has to be done on the 1,100-acre farm. "Cows still have to be milked three times a day," he says.