What is the Reason behind Global Economic Crisis?

In the last few years we have noticed great deterioration in global economic condition all over the world as well as in US. There are various factors that led our economy to this miserable condition. But to understand this current global recession completely, we would have to go deeper.
According to the experts the very basic reason behind this global economic crisis is the downfall of US sub-prime housing market. Alan Greenspan and Bush commented that they are very sorry for this global economic crisis. What Alan Greenspan said in 2003 is very significant" The notion of bubble bursting and the whole price level coming down seems to me as far as a national nationwide phenomenon, is really quite unlikely".
By keeping the entire situation in view, the economic experts have concluded that the main problem is that people are crazy about a house ownership. People believe that housing is the safest investment in these days. Because according to the financial experts, housing has fixed mortgage rates that would not fall easily so they can have great chances of earning interest.
Media is also supporting this idea of housing marketing very much. Media says that housing prices never fall and it could be a very good and profitable business.
But at the same time we saw historically low interest rates in the housing market. This caused great loss to the economy. But according to John Snow the treasury secretary, the idea that we are going to see a collapse in the housing market seems to me improbable."
But the fact is that housing market has now very low interest rates and that is a big reason behind this economic and financial crisis.