Large volcanic eruptions have occurred in Yellowstone approximately every 600,000 years. The most recent of these erupted from two large vents, one near Old Faithful and one just north of Fishing Bridge. Ash from this huge explosion—1,000 times the size of Mount St. Helens—has been found all across the continent. The magma chamber then collapsed, forming a large caldera filled partially by subsequent lava flows. Part of this caldera is the 136-square mile (352.2-square km) basin of Yellowstone Lake. The original lake was 200 feet (61 m). higher than the present-day lake, extending northward across Hayden Valley to the base of Mt. Washburn.