Tourist Jim Lin and his wife, Dafang, stopped to snap a picture of the 306-foot-long Landscape Arch at Utah's Arches National Park June 5, when they were startled by a loud cracking noise. "It was a very big sound, like a dynamite explosion," Lin said. What they heard was a 44-foot slab tearing away from the thinnest section of the arch just before it crashed to the ground. Since then, two smaller rockfalls were witnessed at the arch, one June 13, the other June 21. Park superintendent Noel Poe said it's impossible to predict how long Landscape Arch will continue standing. "It's a natural process we don't fully understand," he said. "It could fall down tomorrow or it could remain a couple hundred years." The largest known rockfall from Landscape Arch occurred in June 1991 when a slab 63 feet long and four feet thick broke off the bottom. The park currently includes 2,000 natural arches, and 42 have collapsed over the last 20 years or so.