What happened Cryptocurrency miners are under pressure today. Three of the largest crypto miners, Marathon Digital Holdings (NASDAQ: MARA) , Riot Blockchain (NASDAQ: RIOT) , and Hive Blockchain (NASDAQ: HIVE) are all down more than 8% as of 2 p.m. ET. Today's marked decline in the crypto mining sector appears to be a direct result of strong regulatory headwinds. Up to now, most of these have come out of China and India. However, it appears U.S. regulators are increasingly looking at crypto mining, from an environmental and sustainability perspective. Putting investors on edge today are reports that Sen. Elizabeth Warren…