Two men face criminal charges for walking onto Old Faithful Geyser in Yellowstone National Park.

Kimberly Guilliams tells KTMF-TV in Missoula, Montana, she saw the men leaning directly over the geyser to take photos Tuesday.

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Going off boardwalks in Yellowstone's thermal areas is dangerous and illegal. Old Faithful erupts with boiling hot water about once every hour.

Park officials say the men have been summoned to appear in federal court. They haven't identified the two men.

Several people have been cited over the years for wandering onto Yellowstone's most famous geyser. Several others have been seriously burned by falling or stepping into the park's thermal features.

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The latest Tweets from GeyserNPS (@GeyserNPS). Predictions for the eruption of Old Faithful Geyser. Yellowstone National Park

Old Faithful FAQ

Source: National Park Service

When does it erupt?

Intervals can range from 60-110 minutes.  

How high does Old Faithful erupt and how long will it last? 

Old Faithful can vary in height from 106-184 feet with an average near 130 feet. This has been the historical range of its recorded height. Eruptions normally last between 1 1/2  to 5 minutes.

How hot is the water in Old Faithful? 

The water temperature at the vent has been measured at 204 degrees Fahrenheit. The steam temperature has been measured above 350 degrees Fahrenheit.

How many gallons of water are expelled during an eruption?

Scientists estimate that the amount ranges from 3,700 gallons to 8,400 gallons, depending upon how long the eruption lasts.