Bitcoin is often called "digital gold," in part because there's a certain amount that can ever be mined. Its scarcity is one reason billionaires turn to the cryptocurrency as a hedge against inflation. But what happens when the last bitcoin is mined? People have already discovered, or unlocked, more than 18 million of that supply. Still, we may be more than a century away from the last coin being unlocked. CNBC's Kate Rooney breaks it all down.