A volcanic caldera is a large depression formed when a volcano erupts and collapses. This occurs after a massive eruption that empties the magma chamber beneath the volcano, leading to the ground above it sinking. Calderas can vary in size and shape, often resulting in a circular or oval basin. These geological features can be found in various locations around the world, such as the Yellowstone Caldera in the United States and the Santorini Caldera in Greece. Over time, calderas may fill with water, creating lakes, or become sites for new volcanic activity.