I think in places like Italy they discover something of that sort almost every single time they dig a hole deep enough into the ground (eg during construction).
Same thing in France, Germany, UK and elsewhere, except the coins are unexploded ordnances.
Some decades ago, swimming in the Mediterranean off the Libyan coast, I dived to pick up what at first very-nearsighted glance (not wearing my specs in the sea) could be an amphora. At the surface it was obvious that it was an unexploded shell, which I put down very carefully then swam far away.
Not sure about developing countries like Libya decades ago, but in places like France, Germany etc. there are well-developed procedures for dealing with this sort of thing, and you should definitely contact the police if you find something like this.
Related… I believe there are many discoveries still to be made beneath the foundations of historic buildings that are too revered to excavate. To do a thorough survey of these sites would amount to taking down the buildings that sit on them, and nobody’s touching those UNESCO-grade buildings, so the secrets beneath remain undisturbed in modern times.
Sometimes you can find a King under a parking lot.