When lahars reach populated areas, they can bury structures and people. An example was the Armero tragedy at Nevado del Ruiz where 23,000 died from an enormous mudflow. Lahars from Glacier Peak pose a similar threat to the small communities of Darrington and Concrete and a lesser threat to the larger and rapidly growing towns of Mount Vernon and Burlington, as well as other communities along the lower Skagit and Stillaguamish Rivers. A 2005 study conducted by the United States Geological Survey identified nine Cascade volcanoes, including Glacier Peak, as "very-high-threat volcanoes with inadequate monitoring". At the time of the study, only one seismometer was installed on Glacier Peak that had not "worked in two years".