By Greg Wood
Business presenter, BBC Radio 4 Today programme
The markets could face many more anxious days yet
Like a stone dropped into a very large pond, the crisis in the US home loans market is sending ripples throughout the world's financial system.
It has already caused a few upsets, an Australian hedge fund collapsing here, a small German state bank having to be rescued there. But September is the month when the true cost of the credit crunch will be felt by some of the richest companies on the planet.
In the glass towers of Wall Street this week, the bankers at Goldman Sachs and Morgan Stanley have been totting up the figures for the past three months.
It has been a good year, with record profits and bonuses. But that is going to stop.
The credit rating agency Standard & Poor's estimates that investment banking revenues could fall by nearly 50% in the second half of this year because of exposure to non-performing mortgages and loans.
In the case of Goldman Sachs alone, that would be a drop of $1.75bn (£852.5m).
Goldman Sachs will report its next set of profits in mid-September and the rest of Wall Street's finest will follow suit. It promises to be a painful experience, and not just for US banks.
Standard & Poor's harsh warning about falling revenue and profits also covers Europe's leading investment banks, including Barclays.
Barclays has had an odd week, and one which illustrates how easily rumour can take hold when investors are scared.
Barclays share price fell because of a story that the bank had exposure to very significant losses incurred by that small German bank mentioned earlier.
Well, it turned out that any exposure is in fact pretty small. But that didn't stop the rumour doing the damage.
Problems in the US housing sector have rippled around the world
And it is not as though these mighty banks were involved in the actual business of providing mortgages to poor Americans with no jobs and no money.
When US interest rates rose and those people started defaulting on their loans, it was a number of relatively obscure sub-prime lenders in states such as California that went bust.
But those lenders were funded by the big banks. And the sub-prime loans were parcelled out to the markets as investments - complex debt vehicles with odd names like "Collateralised Debt Obligations" and "SIV Lites", which are basically bundles of American mortgages that you can buy like a share or a bond.
Nobody wants to buy them now. In the last few weeks these investments have bombed in value - and in some cases they cannot be bought or sold at all.
There is no doubt there are losses out there running into billions of pounds.
Just how much and who's affected will begin to emerge over the next month, and every now and then a hedge fund will collapse.
Students of stock market history are feeling nervous. A summer of tremors has in the past been followed by an autumn crash.
Don't go on holiday in September.