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Yellowstone National Park, Wyoming. Phillips Cauldron, a mud volcano in Norris Geyser Basin that was vigorously active in 1947, intermittently ejecting thick mud clots when surface water was scarce, but a surging gray viscous pool when surface water was more abundant, as shown here, when the pool was about 4 meters in diameter. September 1947. Figure 47, U.S. Geological Survey Professional paper 1456.

Credit: USGS Photo Library

Image 1

YNP RCN, Montana State University, PO Box 173142, Bozeman, MT 59717-3142