The best and most varied area of volcanic geothermal activity on Lassen  is at a location called Bumpass Hell (located six miles from the southwest entrance). The region was named for a Mr. Bumpass (no, really, this is the story) who, in his explorations, stepped through a thin crust over a boiling mud pot and severely burned his leg, ultimately losing the limb.
In fact, the tale of the unfortunate Bumpass makes a good point for travelers visiting the mud pots and fumaroles here: stay on the paths! The dangers at Bumpass Hell are real, and if you step off the boardwalks or let your children run wild, you are risking serious injury and even death.
That said, a hike down to Bumpass Hell will prove fun enough to make the risks worthwhile. You’ll need to walk about two miles from the parking lot and trailhead out to the good stuff—boiling mud pots, fumaroles, steaming springs, and pools of boiling, steaming water cluster here.
Prepare for the strong smell of sulfur, more evidence that this volcano is anything but extinct. Boardwalks meander through the area, creating safe walking surfaces for all visitors.
The spacious parking lot also offers stunning views out east and south, giving you a hint at the scope of the ancient volcano that once stood here. Or if you need to, you can take advantage of the primitive but usable facilities.