New Bern, N.C. — Authorities say a wildfire that started as a controlled burn in the Croatan National Forest has been 70 percent contained.

The blaze was started as a prescribed burn of 220 acres more than a week ago, but it got out of control, burning more than 21,300 acres in the 160,000-acre national forest south of New Bern.

Firefighters are concentrating on fighting ground fires by pumping water into canals around the clock. The soil in the region is full of peat and other organic materials and can ignite easily and burn for weeks.

Officials feared a combination of smoke and extremely moist air could combine to create a super fog that would bring visibilities down to near zero, especially along U.S. 70. But state troopers reported no problems overnight into Sunday morning.

The U.S. Forest Service said that it will investigate why the prescribed burn got out of control. Forestry officials said that as of Thursday, it had cost $300,000 to fight the fire.

The fire sent its odor and particle pollution as far west as the Triangle last week.