Stablecoin issuer, Circle, is experiencing its worst day on record after rival Tether announced it has hired an unnamed Big Four accounting firm to audit its USDT reserves for the first time. This move has caused concern among investors and regulators that the company's reserves may not meet audit standards. Tether's USDT was created in 2014 to be pegged to the value of the U.S. dollar and has been criticized for not providing transparency through quarterly "attestations". Meanwhile, Circle's USDC coin, which undergoes full audits annually by Deloitte and also issues monthly attestations, is seen as more institutional grade than Tether.