US investors are borrowing more money to buy stocks than at any point in history, and that has flashed a warning before every major crash.

• Margin debt — money people borrow to buy stocks — just hit its highest level ever, measured against the size of the economy.
• Every past spike like this came right before a crash: the 1973 crash, Black Monday in 1987, the dot-com bust, and the 2008 financial crisis.
• The real number is even worse, since official data misses leveraged ETFs, options, and other newer ways people borrow to bet on stocks.
• Risky bets are booming — 2x funds on single stocks like Nvidia and Tesla, plus same-day options that are basically gambling on the next few hours.

Outlook: With borrowing at record extremes, the risk of a sharp market drop is higher than it has been in decades.