The Rock Fire was reported shortly after 2:30 p.m., near the Lake Perris State Recreation Area, according to the Riverside County Fire Department.
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Water-dropping helicopters and air tankers slowed the fire's progress.
Fire officials said the blaze had charred 75 acres just after 3 p.m., but by Wednesday night it had grown to 500 acres and was 75% contained.
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It's unknown what caused the fire. No evacuations were ordered.