"Half of the world’s geysers are in Yellowstone. There’s Old Faithful, the most famous geyser in the world, but you’ll also find more than 500 others. Check out Old Faithful—and pop into the visitor center for a multimedialesson on geysers—and then head out along the boardwalk trails to see some of the park’s less-crowded spouters. Riverside Geyser, a cone geyser on the bank of the Firehole River in the Upper Geyser Basin, erupts for 20 minutes every 5.5 to 6.5 hours,shooting a 75-foot stream of boiling water across the river.On sunny days, you can spot a rainbow in its mist. Also in the Upper Geyser Basin, the tallest predictable geyser in the world, called Grand Geyser, erupts every seven to 15 hours, shooting water as high as 200 feet."