The Yellowstone caldera (volcano crater) was formed by three volcanic eruptions. The first, and most powerful, was 2.2 million years ago. The most recent was 640,000 years ago. The caldera encompasses a twenty-eight-by forty-seven-mile area.

The name Yellowstone came from the Sioux, who called the Yellowstone River Mi-tsi-a-da-zi, meaning "rock yellow water."

The early trappers told of bubbling pools of water, steam and boiling water shooting hundreds of feet into the air, and fire leaping from the ground. These stories were treated as tall tales, drunken delusions, and outright lies. An expedition, organized by Ferdinand Hayden, set out in 1871 to provide proof of Yellowstone's incredible landscape. Hayden brought with him the artist Thomas Moran, and a famous photographer of that day, William Henry Jackson. Moran's paintings, and Jackson's photographs, convinced Congress the Yellowstone stories were true.

In 1872, Ulysses S. Grant signed the "Act of Dedication" creating Yellowstone National Park, America's first national park. At more than two million acres, it is one of our largest national parks.

Yellowstone's 10,000 thermal features�hot springs, mud pots, fumaroles, and geysers�total more than the rest of the world's combined. The geysers in Upper Geyser Basin, home to Old Faithful Geyser, form the highest concentration of geysers on earth.

The park is a wonderland of geological phenomena. It is also America's Noah�s Ark. With the successful reintroduction of the gray wolf, every large mammal species that was in the park at its creation is there today.

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