What's 6 miles wide & can end civilization in an instant?

Answer: an asteroid

OK, arguably, meteor or meteorite*.

Informative & funny: How to defend Earth from asteroids. See it at:

http://www.ted.com/talks/phil_plait_how_to_defend_earth_from_asteroids.html

There's an iPad app for this, & other, interesting stuff:

http://itunes.com/apps/tedconferences/ted

* An asteroid (or any other fast moving space object) becomes a meteor (aka falling star) when it enters the Earth's atmosphere.  If it hits the ground, it becomes a meteorite.