A warning to salvagers. Underwater conditions 'arm' iron cannonballs and cookware. Bringing them to the surface can cause them to explode.
When poking through underwater wrecks, the traditional threats to salvage crews and historians have been entrapment in the wreckage, kraken, and vengeful ghosts. It turns out that those who make it out alive are still in danger from the objects they salvage. Iron, when kept underwater for a long period of time, can explode once it reaches the surface, even after being handled.
Davy Jones doesn't like you touching his stuff, and he's booby-trapped it. Instead of harmless stuff like laxatives or superglue, he uses hundreds of atmospheres of pressure. Cast iron is filled with small bubbles the way a cheap chocolate bar is filled with puffed rice. As salts eat away at the cast iron, it becomes porous. Gases become trapped in its many little bubbles. Because they are trapped at the bottom of the ocean, though, they're trapped under enormous pressure. When they're brought to the surface, the pressure on the outside of the pan is released, meaning that instead of being pressed equally from within and without, it's riddled with hundreds of tiny bubbles exerting hundreds of pounds of pressure per square inch. If that pressure isn't released gently - blam.
The saboteurs aren't just age, chemicals and pressure. There are also some living soldiers who make weapons-grade salvage material. As many an unfortunate diver or mermaid lover has found out, there's not a whole lot of air underwater. Oxygen has to be gleaned from other sources.
The ocean contains sulfates, compounds which have oxygen. Bacteria come to live in the crannies of the cannonballs. They take sulfates and denude them of their oxygen, which turns them into sulfides. Sulfides combine with the metal of the cannonballs, but still hunger for oxygen. Since there isn't much on the sea floor, that's not a problem. Once the cannonball comes to the surface, things get messy.
The cannonball commences oxidation. This is a process by which the sulfides grab all the oxygen they can get as fast as they can. When they do so, they take up more room, produce acid, and give off a ton of heat. In other words - blam. With bonus acid spray.
In the past, I may have poked gentle fun at chemists. This is not because I don't respect them. It's just because I think they labor joylessly in labs and college cafeterias over charts and tables and ball-and-stick models. Today I am humbled. The lesson is clear. If you want to go high-seas adventuring, pack a chemist. Or learn to like acid.