Lassen Peak remains an active volcano, as volcanic activity including fumaroles (steam vents), hot springs, and mudpots can be found throughout Lassen Volcanic National Park. Their activity varies based on the season; during the spring, when meltwater is more abundant, fumaroles and pools of water have lower temperatures, while mudpots have more fluid mud supplies. During summer and droughts, they become drier and hotter, since they cannot be cooled by ground water. Geothermal activity can be observed at Bumpass Hell, Little Hot Springs Valley, Pilot Pinnacle, Sulphur Works, Devils Kitchen, Boiling Springs Lake, and Terminal Geyser, as well as the Morgan and Growler Hot Springs south of the national park in Mill Canyon. These are produced by the boiling of underground bodies of water, which generates steam. At Bumpass Hell, these features are at their most vigorous, with temperatures reaching at Big Boiler, the park's biggest fumarole and one of the hottest hydrothermal fumaroles in the world. Because of their acidic conditions and heat, none of these hydrothermal bodies are safe for bathing except for at Drakesbad Guest Ranch. Fumaroles near Lassen Peak in particular remained active through the 1950s, but have grown weaker over time; they can still be found among the volcano's summit craters. These hydrothermal features are monitored continuously for their physical and chemical conditions by the United States Geological Survey.