Yes, old cannonballs can explode

I don't envy the people who lived through the most recent hurricane, but I do envy those who were able to access those beaches afterwards with their metal detectors.  They probably scooped up handfuls of gold doubloons.
But "today I learned" that cannonballs of the Civil War era are supposedly dangerous (?)

A bomb squad was at a South Carolina beach on Sunday after Hurricane Matthew apparently unearthed old Civil War cannonballs from the sand.
Charleston County Sheriff's spokesman Maj. Eric Watson said in a news release that the cannon balls were found on Folly Beach Sunday afternoon, but bomb squad members couldn't get to it immediately because of the rising tide.
Once the ocean level goes down, Watson said technicians would render the cannonballs safe. He warned residents might hear a small boom.