A loophole in the law that prescribes the types of coins that can legally be minted in the US theoretically allows the Treasury Department to mint a $1t platinum coin, deposit it at the Federal Reserve, and then continue paying its bills as normal.
The law includes this clause: "The Secretary may mint and issue platinum bullion coins and proof platinum coins in accordance with such specifications, designs, varieties, quantities, denominations, and inscriptions as the Secretary, in the Secretary's discretion, may prescribe from time to time."
https://www.businessinsider.com/mint-1-trillion-platinum-coin-debt-ceiling-2021-9