Long before any recorded human history in Yellowstone, a massive volcanic eruption spewed an immense volume of ash that covered all of the western U.S., much of the Midwest, northern Mexico and some areas of the eastern Pacific. That climactic event occurred about 640,000 years ago, and was one of many processes that shaped Yellowstone National Park--a region once rumored to be the place where hell bubbles up. Geothermal wonders, such as Old Faithful, are evidence of one of the world's largest active volcanoes. These spectacular features bemused and befuddled the park's earliest visitors, and helped lead to the creation of the world's first national park. Stop at the visitor center to get maps and information. Sorry, pets are not allowed inside.