Let's start with a little background. The mid-2000s economy boomed in part because artificially low interest rates had ignited a housing mania which featured a huge increase in "subprime" mortgage lending. This – as all subprime lending binges eventually do – began to unravel in 2007. The consensus view was that subprime was "peripheral" and therefore unimportant. Here's Fed Chair Ben Bernanke giving ever-credulous CNBC the benefit of his vast bubble experience.
The experts were catastrophically wrong, and in 2008 the periphery crisis spread to the core, threatening to kill the brand-name banks that had grown to dominate the US and Europe. The markets panicked, with even gold and silver (normally hedges against exactly this kind of financial crisis) plunging along with everything else. Gold lost about 20% of its market value in a single month: