Millions of people watch Old Faithful Geyser erupt each year – indeed, it has been attracting visitors since Yellowstone was made the world's first national park all the way back in 1872, the same year this photo was taken.

Old Faithful is one of nearly 500 geysers in Yellowstone, and is one of just six whose eruptions rangers can currently confidently predict. True to its name, the geyser has only lengthened the time between its eruptions by about 30 minutes across the last 30 years.

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