These are boiling mud pots in the Sulphur Works area that you can hop out of your parked car and walk to on the side of the road of Lassen Volcanic National Park Highway, so easily accessible. You can the see the hot steam rising up in the photo, but you really have to be there in person to hear the bubbling and churning and steaming gases and smell the rotten egg odor of the sulphuric acid.
