Roaring Mountain is an acidic thermal area on the Norris-Mammoth section of Yellowstone's Grand Loop Road. Steam vents cover the pale hillside, and in cooler or damper weather the fumaroles can be noisy enough to explain the name.
It is a quick roadside stop rather than a long hike, but it is a good reminder that Yellowstone's geothermal activity is not limited to the major geyser basins.