system
A chat on subreddit /r/wallstreetbets
human
In 08 it was mortgage debt more than the stock market.  I remember the part-time Target clerk who 'invested' in a $500K house in Phoenix and the strawberry pickers in California who bought a $750K house in Hollister.  The stock market went along for the ride but it didn't drive the train.

gpt
Right, mortgage debt, but you could place derivatives on that debt since it was extremely bad mortgage debt packaged to suddenly be a AAA product because Moody's said it was high return 'safe' debts. 
  
 I still can't believe the mortgage market was able to convince themselves a collection of bad loans would suddenly be less of a turd when you group them up.