Supervolcano

When the supervolcano at Yellowstone last erupted 640,000 years ago, the magma and ash ejected from the caldera covered most of the United States west of the Mississippi river and part of northeastern Mexico.Another such eruption could threaten civilization. Such an eruption could also release large amounts of gases that could alter the balance of the planet's carbon dioxide and cause a runaway greenhouse effect, or enough pyroclastic debris and other material may be thrown into the atmosphere to partially block out the sun and cause a volcanic winter, as happened in 1816, the Year Without a Summer. Such an eruption may cause the immediate deaths of millions of people several hundred miles from the eruption, and perhaps billions of deathsworldwide due to the failure of the monsoon[citation needed], as well as destruction of the "American breadbasket", causing starvation on a massive scale