Goldman Sachs, Credit Suisse and RBS borrowed at least $30 billion from the Fed in 2008 via top-secret loan facility that not even Congress kmew about, Bloomberg reported, via the Sydney Morning Herald.

The lending program was called ST OMO (short for single-tranche open-market operations) and in addition to Congress, shareholders and the public were kept in the dark.

ST OMO, which was supervised by the New York Fed, made 28-day loans from March through December of 2008, through an auction process in which the banks bid an interest rate they were willing to pay for the credit.

The Fed held 44 such auctions.

Here's how it worked, according to Bloomberg:

Units of 20 banks were required to bid at auctions for the cash. They paid interest rates as low as 0.01 per cent that December, when the Fed's main lending facility charged 0.5 per cent.

The Federal Reserve Bank of New York, which oversaw ST OMO, posted aggregate data about the program on its website after each auction, said Jeffrey V. Smith, a New York Fed spokesman.

The tally of borrowings is approximately:

Credit Suisse: $45 billion

Goldman Sachs, RBS: $30 billion

Deutsche Bank, Barclays: $20 billion

UBS: $15 billion

Morgan Stanley, BNP Paribas: $10 billion

Citi, JPMorgan, Merrill Lynch: Less than $5 billion

ST OMO was overlooked "when lawmakers required the central bank to publish its emergency lending data last year under the Dodd-Frank law," Bloomberg reports.