Subprime loan crisis in the US

Sub-prime mortgage means the US commerical banks lend money to the poor credit rating home buyers to purchase the houses. The commercial banks sold those kinds of assets to different investment banks which would repackage them to mortgage backed securites (MBS) and sell them to other local and international banks. If the mortgage borrowers can not repay the debt (due to economic downturn), the investment banks would like to unwind their MBS investments. But then, the house price fall will also drag down the MBS value, so many banks contain a lot of 'impaired' or toxic assets which could not be unwinded easily, which impact the financial health worldwide. Therefore, if the Australian banks had bought those kinds of assets, the financial condition would also be deteriorated.

2008-09-29 10:30:22 補充：
Another reason was excessive lending to poor quality borrowers, banks took extra risks in order to gain money.

u gave me a very gd explantion, thz~
but i hv one more question.....=]
so wt was the cause of the sub prime mortgage crisis in the US?? because the mortgage borrowers can't repay the debt and the decline in house prices?? any more factors?