Old Faithful is not the biggest geyser at Yellowstone National Park. That title could be bestowed on one known as Giant Geyser, which can shoot columns of water as high as 300 feet into the air for about an hour at a time. Old Faithful is famous in part because it erupts so frequently and so predictably.
However, Old Faithful's faithfulness isn't the only thing that makes it attractive to visitors. Though it's not the biggest, its eruptions occur approximately every 60-90 minutes and can be 100 feet or more in height. Giant Geyser, on the other hand, erupts rather less frequently and not on a predictable schedule.