For those not up on cryptocurrencies, there are several, of which Bitcoin is the largest by market capitalization and energy use. "Mining" operations race to solve complex math problems, which allows them to validate new entries in a distributed ledger called a blockchain and, in turn, be rewarded with Bitcoin. The system is designed to make calculations increasingly difficult as demand for Bitcoin grows and the supply remains fixed. Solving the calculations requires growing amounts of energy consumed by large data farms. Bitcoin has now passed another significant milestone. According to market data, 90 percent of all Bitcoins set to ever enter supply have now been mined. Ahead of Bitcoin's launch, its pseudonymous creator Satoshi Nakamoto had outlined in a white paper that there would ever be only 21 million Bitcoins. This cap on the upper limit was intended as a way to ensure the coin remains scarce and its value holds steady for years to come.