Reuters' finance blogger Felix Salmon presents the above animated gif, which shows the quantity of high frequency trades happening throughout the day in 14 American stock exchanges, charted between 2007 and 2012.
Watch to the end, to see the enormous spike in activity in 2011, particularly following the US downgrade in August that year, then read Felix's analysis:
Back in 2007, I wasn’t a fan of a financial-transactions tax; today, I am. And this chart shows better than anything why my opinion has changed. The stock market is clearly more dangerous than it was in 2007, with much greater tail risk; meanwhile, in return for facing that danger, society as a whole has received precious little utility.