What was the real root cause of the economic depression/recession of the 2000's?

What it was, is that (at the root of the problem). during the 80's derivatives started to be used in the stock market, these derivatives included the selling of mortgages.

These mortgages (and other loans) would be sold into parts with respective amounts of repayment, due to how much of the loan the one person owned. eventually, lending companies such as Bradford and Bingley thought "if we're just going to sell off our loans unto the stock market, we might as well have no restrictions due to their being no repercussions because we wont own the loan anymore."

So they started handing out loans to people who probably couldn't pay them back, these were called sub-prime mortgages, prime mortgages were ones that were guaranteed the payment back with no default.

Eventually these people didn't pay back their loans, and these derivatives of their mortgage became worthless because, part of nothing is worth nothing.

Due to large banks and investment companies being so highly geared on these sub-primes, they found a large amount of money gone.

So there you go.

Note: I would just like to point out this is a very simplified version. There are a lot of factors that in the end helped this to bring down the whole market, but this was one of the driving factors. I just didn't want anyone to look at this and take it as the entire truth and only reason of why.