[cogitator]

There are several hydrothermal explosion basins in Yellowstone; one of the best is Pocket Basin, a very easy "backcountry" hike not very far from the developed part of the Lower Geyser Basin where Old Faithful Geyser is located. I was able to go there on a special tour about 10 years ago. You do have to be very careful; careless hikers have been burned and have died due to injuries received when they got too close and fell into the thermal features.

[cogitator]

correction: Old Faithful is in the Upper Geyser Basin. Pocket Basin is near the Lower Geyser Basin, which is most famous for the "paint pots" (mud springs).