Deep Impacts and Craters

http://en.wikipedia.org/wiki/Meteor_Crater

http://artbell.com/chances-of-getting-hit-by-asteroid-increasing

Meteor+Crater impacts this size or larger would mean extensive planetary surface devastation.
http://en.wikipedia.org/wiki/Deep_Impact_(film) 1998
Over the past several decades, numerous underground cities & bunkers have been constructed.
http://www.barringercrater.com/about
Unfortunately, most of the surface population is never intended to live in them.
http://en.wikipedia.org/wiki/Impact_event#Sizes_and_frequencies

http://en.wikipedia.org/wiki/Cretaceous%E2%80%93Paleogene_extinction_event#Evidence_for_impact

http://en.wikipedia.org/wiki/Cretaceous%E2%80%93Paleogene_extinction_event#Multiple_impact_event