Millions of visitors each year come to YELLOWSTONE NATIONAL PARK , America's oldest national park and the largest in the lower 48 states, to glory in its magnificent mountain scenery and abundant wildlife, and above all to witness hydrothermal phenomena on a unique scale. Measuring roughly sixty by fifty miles, and overlapping slightly from Wyoming's northwestern corner into Idaho and Montana, the park centers on a 7500ft-high plateau, the caldera of a vast volcanic eruption that occurred a mere 600,000 years ago. Into it are crammed more than half the world's geysers , in which the rain and snow that seep through the bedrock escape the pressure-cooker conditions under the surface in intermittent spectacular blasts, plus thousands of fumaroles jetting plumes of steam, mud pots gurgling with acid-dissolved muds and clays, and hot springs .

Yellowstone amounts to an extraordinary experience, combining the colors of the Grand Canyon of the Yellowstone, limpid Yellowstone Lake, the wild flower meadows and the rainbow-hued geyser pools; the sounds of subterranean rumblings, belching mud pools, and steam hissing from the mountainsides; and the constant smells of drifting sulphurous fumes, with the presence of browsing bull moose, shambling bears, heavy-bearded bison, herds of elk and ubiquitous scurrying marmots . It is, however, very popular; if you let yourself get frustrated by the inevitable crowds and expense, you risk missing something very special. The key to appreciating the park is to take your time, and to plan carefully; above all, try to allow for a stay of at least three days.

More about Yellowstone National Park:

Touring the park

Yellowstone National Park

• Yellowstone National Park
• Touring the park
• Arrival and information
• History
• Winter in Yellowstone

Explore Yellowstone National Park

• Geysers
• Grand Canyon of the Yellowstone River
• Tower and Roosevelt areas
• Yellowstone Lake

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