Exploding cannonballs were hollow iron balls filled with gunpowder, fit with a fuse and lit before they were placed in a cannon and shot toward the enemy target. While often seen in movies, the exploding variety was less common than solid round shot made of stone or cast iron, due to difficulty in timing the explosion and impact, according to Notre Dame Magazine.
Solid shot cannonballs did damage due to their weight and momentum without an explosion. They were useful against fortified walls of castles. At sea, the crew of the ship attached chains and other materials to help take down the enemy ship's rigging. They heated the metal balls to glow red hot with the hope of starting a fire on board the ship, or that the cannonballs would land in the ship's magazine and explode with their stored gunpowder. In battles on the land, the cannon shot the projectiles into columns of soldiers, which led to multiple casualties on impact as well as with each subsequent bounce. A single shot was capable of passing through 40 men and often continued to produce additional gruesome injuries.
While they do not include an explosive load, cast iron cannonballs recovered after years under the ocean sometimes explode. The time underwater, aided by chemical and biological reactions create gases in the porous cast iron. When brought to the surface, the lower pressure of the atmosphere allows the gases to expand, exploding the already rusty cannonball.