"In July, Treasury Secretary Janet Yellen summoned the chair of the Federal Reserve, the head of the Securities and Exchange Commission, and six other top officials for a meeting to discuss Tether. The absurdity of the situation couldn’t have been lost on them: Inflation was spiking, a Covid surge threatened the economic recovery, and Yellen wanted to talk about a digital currency dreamed up by the former child actor who’d missed a penalty shot in The Mighty Ducks. But Tether had gotten so large that it threatened to put the U.S. financial system at risk.
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Tether is what’s come to be known in financial circles as a stablecoin—stable because one Tether is supposed to be backed by one dollar. But it’s actually more like a bank. The company that issues the currency, Tether Holdings Ltd., takes in dollars from people who want to trade crypto and credits their digital wallets with an equal amount of Tethers in return. Once they have Tethers, people can send them to cryptocurrency exchanges and use them to bet on the price of Bitcoin, Ether, or any of the thousands of other coins. And at least in theory, Tether Holdings holds on to the dollars so it can return them to anyone who wants to send in their tokens and get their money back. The convoluted mechanism became popular because real banks didn’t want to do business with crypto companies, especially foreign ones.

Exactly how Tether is backed, or if it’s truly backed at all, has always been a mystery. For years a persistent group of critics has argued that, despite the company’s assurances, Tether Holdings doesn’t have enough assets to maintain the 1-to-1 exchange rate, meaning its coin is essentially a fraud.
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Then, this year, Tether Holdings started putting out a huge amount of digital coins. There are now 69 billion Tethers in circulation, 48 billion of them issued this year. That means the company supposedly holds a corresponding $69 billion in real money to back the coins—an amount that would make it one of the 50 largest banks in the U.S., if it were a U.S. bank and not an unregulated offshore company.

On Twitter, on business TV, and on hedge fund and investment bank trading floors, everyone started asking why Tether was minting so many coins and whether it really had the money it claimed to have. An anonymous anti-Tether blog post titled “The Bit Short: Inside Crypto’s Doomsday Machine” went viral, and CNBC host Jim Cramer told viewers to sell their crypto. “If Tether collapsed, well then, it’s going to gut the whole crypto ecosystem,” he warned.

As far as the regulators are concerned, the size of Tether’s supposed dollar holdings is so big that it would be dangerous even assuming the dollars are real. If enough traders asked for their dollars back at once, the company could have to liquidate its assets at a loss, setting off a run on the not-bank. The losses could cascade into the regulated financial system by crashing credit markets. If the trolls are right, and Tether is a Ponzi scheme, it would be larger than Bernie Madoff’s.
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Tether’s Chinese investments and crypto-backed loans are potentially significant. If Devasini is taking enough risk to earn even a 1% return on Tether’s entire reserves, that would give him and his partners a $690 million annual profit. But if those loans fail, even a small percentage of them, one Tether would become worth less than $1. Any investors holding Tethers would then have an incentive to redeem them; if others did it first, the money could dry up. The bank run would be on."

https://www.bloomberg.com/news/features/2021-10-07/crypto-mystery-where-s-the-69-billion-backing-the-stablecoin-tether