America's hidden gems: The 20 least-crowded national parks in 2009
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19. Lassen Volcanic National Park, California( Russell Virgilio / National Park Service )
Lassen Peak, which last erupted in 1915, is the largest of more than 30 volcanic domes in the park, which is about 165 miles north of Sacramento. Hissing fumaroles (steam vents) and boiling mud pots from those volcanoes continue to shape the park's landscape. Though the climate is unpredictable, summer, with its warm, dry weather, may be the best time for a visit.
Visitors in 2009 (through August): 266,721
Pictured: Boiling Springs Lake