Calderas are among the most spectacular and active volcanic features on Earth. Earthquakes, ground cracks, uplift or subsidence of the ground, and thermal activity such as hot springs, geysers, and boiling mud pots are common at many calderas. Such activity is caused by complex interactions among magma stored beneath a caldera, ground water, and the regional build-up of stress in the large plates of the Earth’s crust. Significant changes in the level of activity at some calderas are common; these new activity levels can be intermittent, lasting for months to years, or persistent over decades to centuries. Although most caldera unrest does not lead to an eruption, the possibility of violent explosive eruptions warrants detailed scientific study and monitoring of some active calderas. Recently, scientists have recognized volcanic unrest at two calderas in the United States, Long Valley Caldera in eastern California and Yellowstone Caldera in Yellowstone National Park, Wyoming. Whether unrest at these calderas simply punctuates long periods of quiet or is the early warning sign of future eruptions is an important but still unanswered question. [Illustration: Yellowstone River plummets through the famous Grand Canyon of Yellowstone. Carved by the river, the “yellow” rocks of the canyon are rhyolite lava flows that have been altered by hot water. The lava flows were erupted after the most recent caldera-forming eruption about 600,000 years ago.] Long Valley Caldera, California. Long Valley Caldera lies on the eastern front of the Sierra Nevada, about 300 kilometers east of San Francisco. A huge explosive eruption about 700,000 years ago formed the caldera and produced pyroclastic flows that traveled 65 kilometers from the vent and covered an area of about 1,500 square kilometers. Ash from the caldera-forming eruption fell as far east as Nebraska. Within the past 40,000 years, eruptions have been restricted to a linear zone of vents, including the Mono-Inyo Craters Volcanic Chain, that extends about 50 kilometers north from the northwest part of the caldera. This volcanic chain consists of many vents that have erupted in the past several thousand years. Eruptions from vents as recently as 550 years ago produced lava flows, pyroclastic flows, and ash, all of rhyolitic composition. Geologic mapping shows that some eruptions were preceded by ground cracking, suggesting that the ground was pulled apart or stretched as magma neared the surface. [Illustration: Sketch of Long Valley Caldera and the Mono-Inyo Craters Volcanic Chain in central California, viewed from the southeast. (_Sketch by Tau Rho Alpha._)] Sierra Nevada Mono craters Inyo craters Mammoth Lakes Resurgent dome Long Valley Caldera