Much of Yellowstone sits inside of a “caldera”, 🌋the exploded crater of a volcano, that last erupted 600,000 years ago. After that eruption, lava flows slowly filled in most of the crater. The water that percolates down into the caldera is heated by that volcanic activity and turns into steam that develops great pressure, forcing it to the surface and producing the park’s geysers, hot springs, fumaroles and mudpots.