Top 10 Things You Didn't Know About National Parks

Not So Faithful?
If you're looking for a reason to trek to Yellowstone National Park, consider this: the geyser dubbed Old Faithful might not be reliable for long. The lapse between steam blasts has lengthened by about 14 minutes in recent years, most likely because of an earthquake in 1983 that altered subterranean water levels. At least four other geysers in the park have been permanently damaged by vandals who threw litter into the geysers' mouths.

• Ken Burns, American Filmmaker