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Looking at times as if they were walking inside a volcano, firefighters battled a fire Thursday night burning on an enormous pile of compost that covers an acre of land at a South Salt Lake business.

The fire spread to two structures and its smoke forced the closure of Interstate 15 and obscured the skies in the Salt Lake Valley.

The fire started around 7:30 p.m. at Diamond Tree Experts at 3645 S. 500 West. It burned a structure at the business and spread to a neighboring business, American West Analytical Laboratories, which does environmental testing.

Kyle Gross, the lab director, said the company had a couple million dollars worth of equipment in the building.

He said there was a fire at Diamond Tree Experts a week ago.

Sixty-three firefighters were attacking Thursday's blaze with numerous trucks and heavy equipment, said South Salt Lake Deputy Fire Chief Kevin Bowman.

The fire was expansive, burning on the top and the sides of a 50-foot pile of mulch that spread across an acre on the Diamond Tree property, Bowman said. Ash floating in the heavy winds were also starting spot fires on neighboring buildings.

Bowman said heavy equipment like excavators would have dig out the compost in order to put out the blaze; he had no idea how long that might take.

Bowman said fire officials were prepared to evacuate businesses, including a dog boarding facility that held between 50 to 60 animals.

By 10 p.m., rain began falling.

Interstate 15 was closed at 3300 South because of smoke the fire obscurring visibility, according to the Utah Department of Transportation. Strong winds were pushing the smoke into downtown Salt Lake City and other parts of the valley.

Firefighters from Taylorsville, Murray, South Salt Lake, Unified Fire Agency, Salt Lake City, Sandy, Cottonwood Heights, Riverton and Draper and West Valley City were on the scene.

By 11 p.m., the fire was still burning.