Many scientists now believe that a "killer asteroid" wiped out the dinosaurs and 70% of all living things 160-million years ago. How likely is it that a similar event can occur again?

The world's largest, most active volcano system exists in the western US. 6,000 years ago, the Yellowstone Volcano erupted. Lava and pyroclastic flows covered 3,000-square miles with ash 3-feet thick. Fossils discovered as far away as Nebraska were found.

Could North America and parts of Europe be headed for a "big chill?" Many experts fear that an abrupt climate change could have catastrophic effects across the planet, including devastating winters in some northern regions.

The floods that destroyed so much of New Orleans after Hurricane Katrina showed how fragile levee systems can be. Now, experts believe a devastating flood could also strike Sacramento.