The past few days have seen a dramatic sell-off in shares around the world, with stock markets seeing falls of the magnitude not seen since the global financial crisis of 2007-08.
Once again, there are worries that the eurozone debt crisis could spread from the bloc's smaller countries, to larger economies such as Spain and Italy.
Investors are also worried that growth in the US economy is slowing, which would have a knock-on impact on the rest of the global economy.
And confidence in the world's largest economy could be knocked after the credit rating agency Standard & Poor's downgraded the US's top-notch AAA rating for the first time in its history.
The world's main stock markets have seen big falls in the past few days, as investors shun shares and look for safer investments.
The falls have wiped out any gains that many of the markets had made this year, with fears growing that we could be heading for another credit crunch.
London's FTSE 100 index fell almost 10% last week, while Germany's Dax lost almost 13% and the Dow in New York fell 5.8%.
Shares in European banks have come under particular pressure as investors are worried about what level of eurozone government debt they are holding, and whether this will be repaid if the eurozone debt crisis spreads.Crisis spreading
One of the main causes of nervousness on the markets has been whether the eurozone debt crisis will spread and about European leaders' ability to deal with it, as voiced by European Commission President Jose Manuel Barroso.
So far Greece, Portugal and the Irish Republic, have already received international help to deal with their crippling debt problems.
Last month, eurozone leaders agreed a second bailout deal for Greece, and also agreed more powers for the European Financial Stability Fund.
This was supposed to have reassured markets that the debt crisis would not spread beyond the "periphery" countries to larger economies such as Spain and Italy.
However, any relief was short-lived, and yields - which indicate the cost of borrowing for a country - on Spanish and Italian debt continued to rise.
Analysts were worried that yields are reaching the point where it would become prohibitively expensive for these countries to borrow on the financial markets and force them to ask for international help, too.
As a result, the European Central Bank has decided to buy Italian and Spanish government bonds to try to bring down their borrowing costs.
The yield on Spanish and Italian 10-year bonds fell shortly after the move.
And the G7 group of industrialised countries has also moved to reassure markets, issuing a statement saying it is "determined to react in a co-ordinated manner".US fears
Another factor which is unnerving the markets is concern about the US economy.
Part of that focuses on the US's ability to repay its debts, with Congress only agreeing on a deficit reduction plan at the eleventh hour, following days of deadlock between Republicans and Democrats over how to raise the US debt limit.
And investors were further unnerved when the credit rating agency Standard & Poor's cut the long-term US rating by one notch from AAA to AA+ with a negative outlook, citing concerns about budget deficits.
There are also worries that the US economy is growing much more slowly than had been hoped.
Last month, figures show the economy grew at an annualised rate of 1.3% in the second quarter of the year, which was slower than expected.
In addition, the growth rate for the first quarter was revised down sharply from 1.9% to 0.4%.
Recent days have brought more gloomy news. Figures showed that US consumer spending fell 0.2% in June, the first drop in almost two years.
However, the unemployment rate fell slightly from 9.2% to 9.1% in July.
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