Fire in the hole

What does it mean when someone said "fire in the hole"?

It is a term used by naval gunners - the old cannons on warships were fired by igniting the powder through a hole on the top of the rear of the barrel which had been muzzle-loaded with powder, shot (either ball or other) and wadding.

the term continued to be used even after breach-loading guns replaced cannon. (breach loaders load from the back and have shells rather than balls - they are not fired by igniting a fire-hole).

It is used to warn other crew that that a naval gun is going to fire, which in a confined space is more dangerous than a normal artillery piece in the open.