What started a fire that burned nearly 3,000 acres in the Ventana Wilderness this summer may remain a mystery.According to Forest Service Public Affairs Officer Andrew Madsen, the cause of the Willow Fire could not be determined.The fire, which started on June 17, burned 2,877 acres in the Ventana Wilderness, west of the Arroyo Seco Campground. It prompted evacuations at the Tassajara Zen Center and Hot Springs. The fire was 100% contained July 12.