A drug dealer who used dark web marketplaces to ply his trade apparently used part of the proceeds to buy vast quantities of Zimbabwean cash.
The FBI said Richard Castro, of Florida, had bought the equivalent of "approximately 100 quadrillion [1,000 trillion] Zimbabwe bank notes" in June 2018.
Use of the currency had ended years earlier, following hyperinflation.
The purchase is thought to have been part of a wider money laundering scheme
Castro also bought hundreds of thousands of US dollars worth of vehicles and "luxury automobile [wheel] rims".
The accused initially made his fortune by illegally dealing opioids, including fentanyl and carfentanil - both of which are stronger than heroin.
These were initially sold via AlphaBay and Dream Market, markets on the Tor network accessed via a special web browser only.
However, after the authorities shut AlphaBay down and were rumoured to have compromised Dream Market's platform, Castro told his clients he would accept purchases via encrypted email only and required them to pay the equivalent of a $104 (£85) fee in Bitcoin to be told the address.
An undercover police officer paid the fee and used the address to order several deliveries, which first helped identify one of Castro's associates and then the dealer himself, who had gone by the nickname Chems_usa.
Castro initially claimed to be not guilty before changing his plea last week.
He has since agreed to surrender a sum worth the equivalent of about $4.16m, earned through the criminal venture.
He is due to be sentenced on 25 October and could face decades in jail.