There are n words in the English language with a double aitch. Touchhold, withheld, and threshhold are three, do you know the nth word?
I’m still in! I’m currently down at Eglin AFB, as a student of NAVSCOLEOD.
Yeah it took me awhile, but I’m here. 
Tripler
I’m in ‘Demo’ class this week.
Yes. It’s ‘En’
Ashhole, on the back of my fireplace.
mangeorge
Good on ya! Just remember, if you have to run, be faster than the instructor…
Well, as for why we use “Fire in the hole!” instead of “Big explosion about to happen, everybody evacuate to a safe distance immedi-BLOOEY”
I dunno.
It’s quick, easy to recognize, and has a very limited number of possible interpretations. It basically gets the message across that everyone that something is about to blow up, what direction it’s happening from (since the guy is shouting and you can figure out where he’s shouting from), and giving you some basic clue as to what course of action you should take (Duck!)
Kinda like why at formation, the sergeants shout “Flight, Ten-CHUT!” instead of “Everybody, stand up straight and pay attention, the Colonel is coming!” Ease of communication.
I haven’t done any research on the matter, but I vividly recall old Western movies set in mining towns (almost certainly made pre-1960, perhaps even earlier?), and they’d yell, “Fire in the hole” whenever blasting was about to occur. Since I’m not a movie maven, perhaps some Cafe Society expert would recall at least one instance? Not making any representations about the accuracy of research done by scriptwriters back then, of course.
zactly
For the same reason we don’t yell “HEY! I JUST HIT A GOLFBALL WHICH MAY OR MAY NOT BE COMING YOUR DIRECTION SO TO BE SAFE IF YOU HEAR THIS YOU MAY WANT TO DUCK!”
Plus tradition, ya know…rum, sodomy, the lash, fire in their holes…
Hitchhiker?
No, that’s the 42nd one.
The standard spelling for that word is “threshold.” The origin of the first part of the word is Middle English “thress” not “thresh.”
In the Royal Navy, when you are about to fire a weapon, you do not give the command ‘Fire’!.
The correct command is ‘Shoot’! With missile systems the command is ‘Engage XXXXXXX’ where ‘XXXXXXX’ is the name of the weapon system.
In the heat of battle, fire may very well be a serious concern, you really want to make sure that there is no chance of mixing up the instructions.
You have a weapon ready to shoot, but if there is a fire and this is yelled, some idiot is likely to set the bloody cannon off.
I think that if this yell of ‘Fire in the hold’ had any maritime connections, the chances are that in the English speaking Navies it would be heavily influenced by Royal Navy procedures. Royal Navy doesn’t use it, never has and has very good reasons for not doing so, so I think that the cannon connection is highly unlikely.
Hole. The word is “hole”.
I’ve never heard anyone yell “fire in the hold”. I’ve never seen that phrase in print (fictional novels), either. Same goes for cannons.
Always and only for setting off explosives.
I don’t remember what the modern american infantry says when they fire an artillary piece, but it’s not "fire in the hole. I think it’s a call and response between the loader and the “trigger man”, to be sure both are ready to fire.
Also, it just doesn’t make sense for such a specific term to have come about - why no “fire in the galley!” or “fire in the fo’c’sle!”? - I think the answer is that to Joe Public, ‘hold’ is thought to mean any area below decks - another quite suggestive indicator that ‘fire in the hold’ is something someone just pulled out of their arse.
Well, it’s very likely that if the US Navy ever did use the term “fire in the hold”, they’d still be using it.
UPDATE:
I was in class today, and we were going over safety steps, to which one of the steps was (paraphrasing)
[sub]Bolding by Tripler. Strangely, I noticed there were no quotes around the phrase in the book.[/sub]
So, it’s fire in the “hole”.
Why? As with the vast majority of all the answers in all of my classes, “Because it’s in the T.O.”
Tripler
And that’s my final answer.