The Yellowstone caldera was created by a massive volcanic eruption approximately 640,000 years ago. Later lava flows filled in much of the caldera, now it is 30 x 45 miles. Its rim can best be seen from the Washburn Hot Springs overlook, south of Dunraven Pass. Gibbon Falls, Lewis Falls, Lake Butte, and Flat Mountain Arm of Yellowstone Lake are part of the rim.