Immensely satisfying, gigantic explosion.
Essentially, "A large container is filled with sawdust, a road flare is stuck in the top and some compressed air is released in the bottom of the sawdust container."
The process is so simple, and the result so incredible, seems too good to be true. So Mythbusters decided to put it to the test. Ben Stone's original video below, Mythbusters' version of the same concept further down.
WonderHowTo make your own gratifying explosions? Lots here.