Studies of the mass extinction event that occurred 65 million years ago have taught us that impact cratering can affect the geologic and biologic evolution of our planet.  It has also heightened the realization that asteroids and comets continue to pose a hazard for life on Earth, including our own species.  The hazards of impact events, both now and in the past, can be assessed by studying impact events in the geologic record.  A good example of the most common type of impact cratering event is Meteor Crater, also know as Barringer Crater, in northern Arizona.  This impact crater is the most pristing impact site in the world.  It was produced 50,000 years ago when a small iron asteroid hit the Colorado Plateau.  When the asteroid hit the ground, it unleashed an extraordinary amount of energy that dwarfs most things in our experience, including the explosive energy of the nuclear weapons that destroyed Hiroshima and Nagasaki.  The impact event excavated millions of tons of rock, redistributed the rock on the land around the crater, produced an intense fireball, and created a shock wave and air blast that devastated plants and animals over hundreds of square kilometers around the crater - all within a few seconds.  This is the type of impact event that affects Earth about once every few thousand years and is the most likely type to occur in the near future.  If an impact event like the one that produced Meteor Crater were to occur today, it could completely destroy an entire city and kill hundreds of thousands, if not millions, of people, particularly if it hit without warning.  Larger, less frequent impact events can produce (and have produced) more severe consequences, including the complete disruption of ecosystems around the world.