Mount St. Helens, a volcano in the Cascade Mountain Range in the State of Washington, USA The volume of material displaced was . Directly in the path of the huge mudflow was Spirit Lake. Normally a chilly , the lahar instantly raised the temperature to near . Today the bottom of Spirit Lake is above the original surface, and it has two and a half times more surface area than it did before the eruption. The largest known of all prehistoric landslides was an enormous submarine landslide that disintegrated 60,000 years ago and produced the longest flow of sand and mud yet documented on Earth. The massive submarine flow travelled – the distance from London to Rome. By volume, the largest submarine landslide (the Agulhas slide off South Africa) occurred approximately 2.6 million years ago. The volume of the slide was . Areas at risk The area most generally recognized as being at risk of a dangerous mudflow are: Areas where wildfires or human modification of the land have destroyed vegetation Areas where landslides have occurred before Steep slopes and areas at the bottom of slopes or canyons Slopes that have been altered for construction of buildings and roads Channels along streams and rivers Areas where surface runoff is directed See also Quick clay, also known as Leda clay Osceola Mudflow, occurred on Mt. Rainier's White River drainage. Citations References . On-line publication 30 November 2013. . Draft of , with page numbers. Further reading . External links United States Department of Homeland Security: Facts about Mudflows/Landslides. Weather hazards Geology articles needing expert attention Landslide types Natural disasters