Anyone who watches TV, especially Mythbusters, will have heard the expression “Fire in the hole”. It is usually called out before a blast or explosion is deliberately detonated, warning those present to take cover.
Having wondered why someone pushing down the plunger on a detonator that looks like an upright shoebox would shout out “Fire in the hole” rather than “Watch out” or something similar, I looked it up.
It turns out that it is a leftover from the days when armies fought by walking towards each other into cannon fire and rifle shot, playing drums and fifes. You can see such battle scenes in Barry Lyndon.
The cannon in those days were simple. You took a big metal tube and put gunpowder into it. Then you put in a cannonball or lots of metal shrapnel. Then you put wadding down the tube, tamping it down with a big stick with a big lump on the end (impressed with my military jargon and technical talk?).
There was a little hole at the tail end of the cannon that was called a nipple. Don’t ask me why a hole is called a nipple, I don't know. God knows what the business end of the cannon was called, no doubt something to do with its phallic shape.
When the cannon was about to be fired, it was important to let those around know so that they wouldn’t be standing in front of the cannon at the inappropriate moment or even behind it, since those cannon had a habit of being propelled backwards when the charge was ignited. When the gunpowder was ready for ignition by pushing a lit stick into the small hole at the base, the person in charge yelled “fire in the hole”, both as a command to light it and as a warning to those nearby. The command to light it eventually became “Fire” and is still used in this context for discharge of weaponry; the expression “fire in the hole” became a general warning for explosive weapons and from there explosives generally. It is now also used in mining and especially by special effects departments when an explosion or loud sound is imminent.