Upper Geyser Basin, Yellowstone National Park
The highest concentration of geysers and hydrothermal springs in the world are located here in this corner of Yellowstone National Park, called the Upper Geyser Basin. They include what is perhaps the most famous geyser of them all: Old Faithful. But the Upper Basin contains many other geysers as well, including the tallest predictable geyser (Grand Geyser) and the most voluminous geyser (Giant Geyser). Yellowstone contains about 300 geysers, about two-thirds the number in the entire world.
Geysers are essentially a rare form of hot spring—a water-filled tube that extends thousands of feet into the Earth's crust, so deep it makes contact with molten rock called magma. The water in the tube boils and under extreme pressure ejects the water column into the air, emptying the tube. After some time, more groundwater seeps into the tube, filling it, and starting the process over again. That's why geysers erupt at somewhat regular intervals. The bigger the tube, the more water, and the longer the eruption. Yellowstone is one of the few places in the world where you can safely walk among so many geysers and superheated springs, and view them close up. Trails and boardwalks guide the way, making the Upper Geyser Basin one of the star attractions of this famous national park.