Signed into law as the world's first national park on March 1, 1872, Yellowstone holds over half of the world's geysers and, since the re-introduction of wolves in 1995, the full complement of wildlife that was present when the park was formed, including bison, grizzly and black bears, elk, white-tail and mule deer, river otters, coyotes, and, of course, wolves.
At 3,468 square miles in size, Yellowstone National Park is one of the largest national parks in the lower 48 states and is larger than Delaware and New Jersey combined. It boasts the most remote location in the continental U.S., hundreds of miles of trails, and world-renown catch-and-release trout fishing.
The park itself sits on top of one of the most active volcanoes in the world and is considered a "supervolcano", the last major eruption of which was over 1,000 times as destructive as the 1980 Mount St. Helens eruption. Yellowstone also has one of the world's largest petrified forests. There are 290 waterfalls in the park fifteen feet or higher, the largest of which is the famous Lower Yellowstone Falls at 308 feet in the Grand Canyon of the Yellowstone.
Yellowstone's most famous geyser, Old Faithful, erupts every 90 minutes or so to a height of up to 185 feet. The world's highest geyser, Steamboat Geyeser, is located at Norris Geyser Basin and has been recorded throwing water over 300 feet in the air.
For more information, see Yellowstone's Official Web Site.