Located in east-central California, the Long Valley Caldera comes in a close second only to Yellowstone, in terms of sheer size. Measuring in at close to 200 square miles, it is also one of the largest active volcanoes in the world. The Long Valley Caldera last erupted over 760,000 years ago, spreading ash as far as Nebraska. In the early 1990′s, large amounts of carbon dioxide began seeping through the ground killing the surrounding vegetation. These warning signs are alarming to scientists, as it points that an eruption may happen in the near future. Although, in terms of the near future to mother nature, that may mean, centuries, decades, or just merely years away.