Fire in The Hole

"Fire in the hole" is a warning that an explosive detonation in a confined space is imminent. It originated with miners, who needed to warn their fellows that a charge had been set.

The first cannons developed were discharged, shot or exploded by placing a flaming torch to a small hole packed with gunpowder and leading to the main powder charge. This caused the main charge to explode, propelling the cannon ball to the enemy, or sometimes, blowing up the cannon and all standing nearby.

Hence, fire in the hole was both a command to the torch man, and a warning to all around. Over time cannons improved; they became safer, with no hole or fire needed. The command was reduced to fire, while the full phrase fire in the hole became a general warning for the use of explosive weapons. It was subsequently adopted by the United States Army and Marines to give notice that a grenade or satchel charge was being tossed into a bunker, spider hole, or other enclosure. The term frag out is also commonly used. It is not used for all explosions – throwing a grenade in the open is not announced, for example – only those surprisingly close.