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CUSTER, S.D. (AP) — A wildfire in a popular South Dakota state park grew rapidly overnight.

The fire in Custer State Park, which is just south of Mount Rushmore National Memorial in the state’s southwest, grew from about 6 square miles on Tuesday to about 55 square miles on Wednesday and is 7 percent contained, according to park officials.

“From hearing the briefing this morning, a lot of these firefighters say they haven’t seen anything like this fire in 10 to 20 years,” said Kobee Stalder, the park’s visitor services program manager. “A lot of people are comparing this to a midsummer fire.”

No one has been injured by the fire, which started Monday. Fences and outbuildings have burned, but none of the park’s main buildings has been destroyed.

The fire’s cause is unknown.

Crews succeeded in protecting the popular State Game Lodge by burning downed trees and grass between it and the fire.

The park is home to a herd of about 1,300 bison, and park officials tweeted Wednesday that they’d be checking on the herd and other animals along its popular wildlife loop road.