Nearly £60 billion has been wiped off the values of the FTSE 100 since the start of August as the upbeat mood elsewhere in the economy fails to lift the stock market.
Optimism over growth, a buoyant property market and surging retail sales have failed to spur on the index of London's leading shares after it surged close to an all-time high earlier in the year.
Traders are instead looking ahead to what might happen if prospects to continue to improve - and are fearful about what they see.
The index of London's leading shares reached 6682 at the close on August 1 but were more than 200 points off to 6453.5 at the end of today's session, representing a drop in value of £58.4 billion for the month so far.
Three months ago in May, the FTSE 100 had reached 6840.3, its best result since the dotcom boom in December 1999.
Even now, the index is still nearly 1,000 points above its level last summer when it sunk below 5,500 points.
Markets have been propped up by massive asset purchase programmes - or quantitative easing - by central banks leaving many anxious about what will happen when this is taken away.
There are increasing expectations that the US Federal Reserve will begin to "taper" its QE from next month.
It means that better-than-expected economic data from the world's largest economy may perversely be received with dread because it increases the likelihood that policymakers will see less of a reason to keep monetary stimulus in place.
Quickening growth in the US and the end of the recession in the eurozone have therefore not been as welcomed as might be expected.
Chris Ralph, chief investment officer at St James's Place Wealth Management, said: "Markets have reacted to the good news as if it were bad.
"Global equities slid last week as traders took the positive economic data as likely to bring an earlier rise in interest rates and a reduction of easy money."
The reaction in the stock market has implications that go wider than the trading floors in the City or Wall Street.
Central bank purchases of Government bonds have pushed down the yields investors can expect from them but fears that they will pull out have caused yields to rise.
Higher yields offered on bond markets ultimately push up rates that banks offer to borrowers such as homeowners looking for mortgages.
The turmoil has also rippled out to emerging markets as investors become more anxious about where they put their money. India has been particularly badly hit, with a plunge in the rupee amid concerns over the government's economic management.
Events across the Atlantic have a strong pull on the mood in London because many of the UK's top-listed companies are heavily focused on the US.
But Britain also has its own £375 billion QE programme, backed up with historically-low interest rates of 0.5%, to worry about.
Traders had been reassured by a strong early hint from new Bank of England governor Mark Carney that interest rates would remain low for some time to come, and the promise of a new policy of "forward guidance".
But though it was designed to reassure markets about the future path of rates, it instead had the opposite effect when it was announced earlier this month.
Markets had already "priced in" the expectation that interest rates would be held down for years to come, but a series of caveats built into the policy in case of inflation stoked fears that it might end sooner than had been thought.