… conditions on the sea bed, surrounding by salty, oxygen-rich water, allowed the iron balls to rust almost all the way through. This would have caused them to expand, making them less dense. Indeed, the recovered balls were much lighter than expected.
At the same time, the cannonballs were gradually sinking into the sand, which reacted with the metal to form an airtight seal around them. Over the decades, rotting organic matter that was entombed along with the oxidized metal converted it back into pure iron.
Crucially, the volume remained the same - leaving pores where the oxide had been. When the seal was broken, air permeated these pores, with dramatic results.
Corrosion researcher Stephen Fletcher at Loughborough University, UK, explains that it is not unusual for finely divided iron to heat up rapidly when exposed to air. As iron oxidizes it releases energy, and the huge surface area of the fine filaments means that this happens so quickly that they can actually burn.
Hand-warming devices for Arctic explorers, round-the-world sailors and British holidaymakers exploit this phenomenon. “But you just don’t expect a solid cannonball to be made up of finely divided and compressed iron powder,” says Rosseinsky.