The stock market, and not just the US stock market, has taken several serious dives based solely on the problems caused by the way the subprime loans were made by one bank, then bundled as super safe secured loans and peddled around the world as great high yield investments. The banks that issued the loans sold them to others, and when the variable interest rates were raised, and defaults began occurring it set of several bad cases of panic.
Surely you have heard of this on the news.