The correct answer is Housing.
The financial crisis of 2008 is considered by several economists the worst economic disaster since the Great Depression of 1929.
It began in 2006 when housing prices started to fall. At the time realtors saw as a positive sign such as an overheated market reaching a more sustainable level.
What realtors did not realize back then is that there were too many homeowners with questionable credit, for example, loans for 100% or more of the value of the house, which led banks to invest in subprime areas (mortgage-backed securities including bundled loan portfolios, derivatives and credit default swaps).
This type of investment mentioned above collapsed the investment bank Lehman Brothers on September 15th, 2008 developing an international banking crisis.