Two glacier-fed rivers join forces at the foot of the massive 4,392 m (14,411 ft) Mt. Rainier, potentially one of the most dangerous volcanoes on the planet. Washington State, USA. Rainier is highly glaciated, meaning that an eruption would send destructive mud-flows called lahars, fueled by the melted glacial ice, racing down multiple rivers including the Puyallap River which flows right into the port of Tacoma. Lahars from past eruptions, the most recent being in the 19th century, have reach the sea at Puget Sound, and the USGS says that as many as 150,000 people now live in communities that are actually built upon past lahar flows. The upper part of the mountain, topped by two volcanic craters and an unseen crater lake under the snow, experts say is slowly being weakened from hydrothermal activity. This means that the summit could collapse like its neighbor Mt. St. Helens did in a 1980 eruption but with exponentially worse consequences because Rainier and its glaciers are much larger than St. Helens. It has been designated one of 16 "Decade Volcanoes" scattered around the world by the International Association of Volcanology and Chemistry of the Earth's Interior (IAVCEI), because of its proximity to a major metropolitan area, Seattle-Tacoma, and its history of destructive eruptions. Although Rainier is not quite the tallest mountain in the lower 48 states, it is topographicaly the most prominent mountain mass, as it basically rises from sea level to its full height.