Until money markets go wrong, the rest of us barely know they exist.
But something has gone seriously awry in the interbank market, which is where banks lend colossal sums to each other.
The measure of what's gone wrong is the record gap of almost 1 ½ percentage points - or 145 basis points to be precise - between the interest rate for lending between banks in sterling for three months (what's called three month Libor) and the market's expectation of the average overnight interest rate for the coming three months (or the three month OIS rate).
The gap has never been as wide as that, though it has periodically been over one percentage point since the credit crunch began last August.
Before the crunch, the average gap was 0.1 percentage points (10 basis points).
What that gap shows is that banks are happy to lend for 24 hours, but not for any longer than that.
In fact, banks have more money to lend overnight than they know what to do with.
They are depositing a ton of it with the Bank of England in its standing deposit facility, which pays a paltry penal interest rate of 4%.
Think about that for a second.
Our banks are prepared to lend to the Bank of England overnight at 4%, but not to each other for three months at more than 6%.
What on earth is going on?
Well the background is that banks are rebuilding their balance sheets to cope with the economic downturn we're experiencing, and they're doing that by lending less (and raising new capital).
But the more immediate cause is a sudden flare up - post the debacles at Lehman, AIG and the rest - of fears that no financial institution is safe from collapse.
So bank chief executives and treasurers think it's wiser to hoard cash (or liquidity) than to lend it out, even if that leads to a reduction in profits (which it does).
Also, there's a significant potential funding problem for all banks in the growing risk aversion of US money market mutual funds, which are increasingly reluctant to lend their trillions of dollars for more than a few days at a time (because they were burned on Lehman and because their shareholders are withdrawing cash on a significant scale).
The rise in interbank rates for lending longer than overnight is the most palpable sign of crisis in the global banking system, a crisis engendered principally by fear.
Banks aren't fulfilling their core function, of transmitting money to where it's needed.
It's why Hank Paulson and Ben Bernanke may not be guilty of hyperbole when they claim that their $700bn banking bailout plan may be the difference between life and near death for the global financial economy.