From 2009 to 2011, the S&P 500 was worth an astonishing $10.4 trillion, according to FactSet.
But by the end of 2012, the index was down to $2.9 trillion, or just a little over half of what it had been in January 2019.
It has since recovered to about $1.8 trillion.
So what is behind the rally?
What caused it?
And how does it compare to other markets?