Yellowstone National Park spans 3,468 square miles spanning lakes, canyons, rivers and mountain ranges. Yellowstone Lake is one of the largest high-altitude lakes in North America and is centered over the Yellowstone Caldera, the largest super volcano on the continent. The caldera is considered an active volcano. It has erupted with tremendous force several times in the last two million years. Half of the world's geothermal features are in Yellowstone, fueled by this ongoing volcanism.
Hundreds species of mammals, birds, fish and reptiles have been documented, including several that are either endangered or threatened. Grizzly bears, wolves, and free-ranging herds of bison and elk live in the park. The Yellowstone bison herd is the oldest and largest public bison herd in the United States. Paved roads provide close access to the major geothermal areas as well as some of the lakes and waterfalls. During the winter, visitors often access the park by way of guided tours that use either snow coaches or snowmobiles.