"Pro of my job: being asked by museums to evaluate the condition of their Civil War ordnance.
Con of my job: finding one that is still live."
That hole is where a timed fuse is inserted. Below that is a plug which keeps the black powder and shrapnel balls packed inside the shell.
That plug is still in place, meaning that 150 years of powder offgassing has built up an immense amount of pressure inside the shell. It's at no risk of exploding from combustion, but right now it's much like a pressurized air cylinder. Strike it hard enough and in the right place and you're going to be the Civil War's latest casualty.
Holy Crap! :o
Needless to say, BE CAREFUL.