Twenty-five years ago I spent my summers beside sulfur-belching hot springs in northern California. The hot springs were not as big as Yellowstone’s. Most were just a few feet across, one or two about a dozen feet wide. None of them were truly boiling, but they were hot to the touch and gases bubbled vigorously out of them.
To add to the general ambience of roasting sulfur, air temperatures in that part of California each July and August are in the 100-degree range, and in addition to sulfur, the hot springs carried a lot of mercury, arsenic and other toxic metals.