The 1.5-mile-long Bumpass Hell Trail in Lassen Volcanic National Park connects its trailhead parking lot at the Lassen Park Road and nearby Lake Helen with Bumpass Hell. This is a boardwalk-accessible hydrothermal area of hot springs, steam vents and mudpots occupying the old eroded vent of a dormant dome volcano—Bumpass Mountain, whose present peak can be seen half a mile north.
The first half of the trail is a well-graded path east of Little Hot Springs Valley. This section with a negligible elevation gain of 200 feet takes you to a saddle with a west-side overlook offering views of Mt. Conard, Diamond Peak, Brokeoff Mountain, Mount Diller and Pilot Pinnacle. An interpretive panorama panel puts the current landscape geologically into context with once towering Mt. Tehama—before it collapsed and present day Lassen Peak, a plug dome volcano, took over its landmark role.
The second half of the Bumpass Hell Trail leads over the saddle and downhill into the hydrothermal area to the east of the saddle. Along this section you'll find an interpretive panel introducing the area's namesake: ill-fated cowboy-prospector Kendall Vanhook Bumpass (1809-1885). Descending further, you'll come to a junction, at which the boardwalk over the brittle crust of the geothermal field starts, while along its south margin a hiking trail continues to Cold Boiling Lake (1.9 mi, 3.0 km), Crumbaugh Lake (2.4 mi, 3.8 km) and Kings Creek Picnic Area (2.5 mi, 4.0 km). From Crumbaugh Lake, you can continue west to Mill Creek Falls and the Kohm Yah-mah-nee Visitor Center at Lassen Volcanic's southwest entrance.
If you don't mind whiffs of rotten-egg smell and an occasional breeze of hot steam from a fumarole, you may want to stroll along the educational boardwalk and explore—close-up—the hydrothermal area, which Tracy Salcedo-Chourré described as follows :
Bumpass Hell's evocative name suits it perfectly. Like its moniker, this geothermal area is a combination of the whimsical and the ominous. Fantastically colored superheated water swirls and bubbles in large pools, and burping mudpots are endlessly entertaining, but columns of hot steam and the wickedly rotten scent of volcanic gases (not to mention the numerous warning signs posted alongside boardwalks) are vivid reminders of the violence of the area.
You probably recognized one of those entertaining mudpots in the photo above. Along the boardwalk interpretive panels explain the working of other hydrothermal features including the Big Boiler, a fumarole with steam temperatures as high as 322 F (161 °C), the Boiling Pool with bacteria living in its hot acid-sulfate water, and the Pyrite Pools with black scum—a frothy mass containing tiny crystals of the iron-sulfide mineral pyrite—floating on its surface. Yellow sulfur and white-yellow sulfate salts are found scattered all over the thin crust surrounding the pools and holes. At the end of the boardwalk you'll arrive at the turquoise pool shown below. Here, the less heated ground and cooler water allow for the growth of plants including mountain heather and bog-laurel.
 Tracy Salcedo-Chourré: Hiking Lassen Volcanic Park. Globe Pequot Press, Guilford, Connecticut, 2001; page 40.