As some people already mentioned, the problem is the amount of cash you would have to keep safe somewhere. As far as I know, there are companies that use tunnel

As some people already mentioned, the problem is the amount of cash you would have to keep safe somewhere. As far as I know, there are companies that use tunnels in mountains to act as huge cash depots. But such a storage also comes at a price. Also to prevent this kind of business, the European Central Bank already considered dismissing the 500 Euro bill. With that the physical amount to store would be even bigger. (The german economics professor Hans-Werner Sinn mentioned this once in a talk)