The obligatory Old Faithful shot
While this is the most famous geyser in the park, it is only one of approximately 250 geysers, and one of about 10,000 thermal features.  It is not as "faithful" as it once was, with eruptions every 35 to 120 minutes.  The timing of the next eruption can be closely predicted by the length of the previous eruption, usually 1.5 to 5 minutes.

Upper Geyser Basin near Old Faithful 

Yellowstone is one of the world's largest calderas (volcanic explosion crater), 28 miles X 47 miles.  That is Old Faithful Lodge in the upper right.  Built in 1904, it is a National Historic Landmark.

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