Investors have borrowed more money to buy stocks than at any point in US history, which is bad news for anyone holding stocks when the market turns.

• Margin debt just hit its highest level ever compared to the size of the economy — above the dot-com bubble, 2007, and the 2021 meme-stock craze.
• The real number is likely much worse, because official data only counts old-fashioned broker loans.
• Leveraged ETFs, options, portfolio margin, and private credit all add hidden borrowing that never shows up in the stats.
• Funds that double the daily move of single stocks like Nvidia and Tesla are pulling in hundreds of billions, and same-day options are trading at record volumes.
• The same kind of leveraged funds that wiped out Korean investors are widely available in the US.

Outlook: Heavy borrowing makes any market drop far more violent, so the downside risk keeps building as long as leverage climbs.