A half-mile wide, rain-soaked ridge became a mudslide Sunday in rural Colorado and has rescue workers searching for three men who were surveying an earlier mudslide.

The video above, released by the Mesa County Sheriff's Department, shows the tree line where the mudslide began. The department is warning that the area is still volatile.

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The mudslide occurred in near Grand Mesa, one of the world's largest flat-topped mountains, reports AP.

Sheriff's deputies suggested the ridge had been moving most of Sunday, and a caller reported the mudslide around 6:15 p.m. saying it sounded like a freight train.

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The search for the men has been difficult, because only about third of the area where the slide occurred is stabile. The landslide is 20-30 feet deep at the edges and could be as much as several hundred feet deep in parts.