The pair proved that about 5,600 years earlier, the summit of Mount Rainier had collapsed and caused a landslide filling some valleys up to 400 feet deep. That awakened the recognition that a similar event could endanger hundreds of thousands of people living atop the ancient mudflows. They went on to write the 1978 report that claimed Mount St. Helens is an especially dangerous volcano and it would more than likely erupt before the end of the 20th century.