Mud Volcano Loop Trail, Yellowstone National Park - September 15, 2007

“Frothing and Fuming as heat and gas rise from Yellowstone’s magma chamber, this muddy pool churns and cooks. Shaken again and again by earthquakes, the temperature beneath it rises and falls, transforming Churning Caldron.” – park sign

While hot, this thermal feature is not boiling.  It’s temperature is probably in the range of 150°F to 170°F.  The violent churning is due to gases escaping from below.