Yellowstone NP is known for its many hydrothermal features with Old Faithful Geyser being the best known. At the heart of Yellowstone’s past, present, and future lies volcanism. Huge volcanic eruptions started two million years ago. The latest eruption caused the central part of what is now the park to collapse forming a 30 to 45 mile caldera ( basin). The magnetic heat powering those eruptions still powers the geysers, hot springs, fumaroles, and mudpots.