What happened Cryptocurrency miners are under pressure today. Three of the biggest crypto miners, Marathon Digital Holdings (MARA -15.14%), Riot Blockchain (RIOT -13.13%) and Hive Blockchain (HIVE -9.33%) are all down over 8% at 2 p.m. ET. Today’s marked decline in the crypto mining industry appears to be a direct result of strong regulatory headwinds. So far, most of them have been from China and India. However, it appears that US regulators are looking more and more at crypto mining, from an environmental and sustainability perspective. Today, investors are on the lookout for reports that Senator Elizabeth Warren sent a…