Firefighters on Johns Island corralled a stubborn brush fire Tuesday morning that burned underground and crept within 75 feet of homes on Island Estates Drive.

Capt. James Ghi of the St. Johns Fire and Rescue Department said the wildfire that grew to 20 acres started in a pile of mulch. Crews were summoned around 7 a.m.

At first sight, St. Johns crews knew the blaze’s size would require help from Charleston and the S.C. Forestry Commission.

Ghi said most of the flames were limited to 3 to 4 feet in height as they burned through underbrush, though some reached the treetops. Battling sustained winds of 20 to 25 mph, firefighters stopped the fire from nearing homes in the area just northwest of Main and Brownswood roads.

“We did a really good job of surrounding it, and the bulldozers cut lines around it,” Ghi said. “There was a potential for those homes to be threatened.”

Parts of the blaze burrowed into the ground, which produced a lot of smoke and less visible fire. Its cause was not known and will be difficult to investigate, Ghi said.