X-raying the earth reveals a vast fault 80 miles off the Pacific Northwest coast. For over 300 years pressure has been silently building. When it slips if will generate a megaquake and tsunami that will devastate the region threatening the lives of 15 million residents living in Cascadia. Now scientists are racing to understand how bad the destruction will be when the really Big One hits.

A large volcano beneath Yellowstone National Park could plunge Earth into a volcanic winter.