Okay, as structural engineers, most of us are in the business of getting structures to stand up; not fall down. However, there is a special niche of the industry whose job it is to make structures come down quickly and safely. Unlike most structural engineers who long, hard hours of work go to making something stand up for years (decades? centuries, even?), their work pays off in fractions of a second.
Controlled demolition usually involves prelimary weakening of the structure so it essentially will just support itself under service conditions. Then, explosive charges are set through out. Now, these aren't really there to 'blow the building up.' Rather, they serve as knives to cut many load-bearing elements simultaneously (and with no people around). Once the charges go off, the structure gets sliced up into nice chunks which come down. Further, through some fancy timing, the structure can be made to fall in as safe a manner as possible. Hence: controlled demolition.
Now, in case you hadn't guessed by glancing down at the bottom of this blog post, the title relates to the following video produced by VDOT (that's the Virginia Dept. of Transportation). Explosive demolition of old bridges as set to the Barber of Seville. Enjoy:
As a former bridge engineer who lived in Virginia, I can say that this video was especially fun for me to watch.