One of Lassen Volcanic's most active thermal areas is "Bumpass Hell" (left), named after an early explorer who lost a leg to one of the hot pools. Bumpass Hell is an easy 1.25 mile hike from the main road, with about at 300-foot elevation drop into the "hell" itself.

Highlights include "Big Boiler" (top right, steaming in the background), a loud, sulfur-smelling fumarole; and Hot Lake, left. Below left, another pool splashes hot murky water, while a mud pot bubbles, right.

Left, a mud/steam vent at Sulfur Works, a small thermal area accessible from the main road through the park.