fed chair ben bernanke explained in 2010 that vulnerabilities in the global financial system built up over a long period of time, and then specific triggering events set the 2007-2008 subprime mortgage crisis into motion. for example, vulnerabilities included failure to regulate the risk-taking of the non-depository banking sector, the so-called shadow banks such as investment banks and mortgage companies. these companies had outgrown the regulated depository banking sector, but did not have the same safeguards. further, financial connections were established between the depository banks and shadow banks (e.g., via securitization and special purpose entities) that created dependencies that were not well understood by regulators. certain types of derivatives, essentially bets on the performance of other securities, remained largely unregulated and were another opaque source of dependencies.