Yellowstone's Upper Geyser Basin contains the Old Faithful geyser which is Yellowstone's most famous feature. But the park contains many other spectacular geothermal, geological, and animal features. Perhaps most impressive is the ancient caldera, or volcanic cone, that formed Yellowstone 640,000 years ago. The eruption that occurred then left a caldera roughly 40 by 60 miles wide and most of these photos were taken from inside that caldera.