Bitcoin miners get 6.25 bitcoin ($129,000) for every block they verify, while Ethereum miners get 2 ether ($2400) plus gas, which are the fees users pay on each transaction. Until last week, miners couldn’t cheat this system because most participants had to agree that, yes, each block represented a true and accurate history of the blockchain. Given the enormous amounts of money that can be made from mining, those who can afford it establish enormous mining farms, using hardware that produces higher computational power, to try to verify more blocks than others, which yield them more cryptocurrency rewards. They also...